diff --git a/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt b/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt
index 7fda147a..76a0a806 100644
--- a/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt
+++ b/app/src/main/java/com/stevesoltys/seedvault/repo/Checker.kt
@@ -6,6 +6,7 @@
 package com.stevesoltys.seedvault.repo
 
 import androidx.annotation.WorkerThread
+import com.google.protobuf.ByteString
 import com.stevesoltys.seedvault.backend.BackendManager
 import com.stevesoltys.seedvault.crypto.Crypto
 import com.stevesoltys.seedvault.proto.Snapshot
@@ -71,10 +72,12 @@ internal class Checker(
         this.handleSize = handles.size // remember number of snapshot handles we had
 
         // get total disk space used by snapshots
-        val sizeMap = mutableMapOf<String, Int>()
+        val sizeMap = mutableMapOf<ByteString, Int>() // uses blob.id as key
         snapshots.forEach { snapshot ->
             // add sizes to a map first, so we don't double count
-            snapshot.blobsMap.forEach { (chunkId, blob) -> sizeMap[chunkId] = blob.length }
+            snapshot.blobsMap.forEach { (_, blob) ->
+                sizeMap[blob.id] = blob.length
+            }
         }
         return sizeMap.values.sumOf { it.toLong() }
     }
@@ -96,13 +99,13 @@ internal class Checker(
             "Got $handleSize handles, but ${snapshots.size} snapshots."
         }
         val blobSample = getBlobSample(snapshots, percent)
-        val sampleSize = blobSample.values.sumOf { it.length.toLong() }
+        val sampleSize = blobSample.sumOf { it.blob.length.toLong() }
         log.info { "Blob sample has ${blobSample.size} blobs worth $sampleSize bytes." }
 
         // check blobs concurrently
         val semaphore = Semaphore(concurrencyLimit)
         val size = AtomicLong()
-        val badChunks = ConcurrentSkipListSet<String>()
+        val badChunks = ConcurrentSkipListSet<ChunkIdBlobPair>()
         val lastNotification = AtomicLong()
         val startTime = System.currentTimeMillis()
         coroutineScope {
@@ -116,7 +119,7 @@ internal class Checker(
                         } catch (e: Exception) {
                             log.error(e) { "Error loading chunk $chunkId: " }
                             // TODO we could try differentiating transient backend issues
-                            badChunks.add(chunkId)
+                            badChunks.add(ChunkIdBlobPair(chunkId, blob))
                         }
                     }
                     // keep track of how much we checked and for how long
@@ -154,25 +157,30 @@ internal class Checker(
         checkerResult = null
     }
 
-    private fun getBlobSample(snapshots: List<Snapshot>, percent: Int): Map<String, Blob> {
-        // split up blobs for app data and for APKs
-        val appBlobs = mutableMapOf<String, Blob>()
-        val apkBlobs = mutableMapOf<String, Blob>()
+    private fun getBlobSample(
+        snapshots: List<Snapshot>,
+        percent: Int,
+    ): List<ChunkIdBlobPair> {
+        // split up blobs for app data and for APKs (use blob.id as key to prevent double counting)
+        val appBlobs = mutableMapOf<ByteString, ChunkIdBlobPair>()
+        val apkBlobs = mutableMapOf<ByteString, ChunkIdBlobPair>()
         snapshots.forEach { snapshot ->
             val appChunkIds = snapshot.appsMap.flatMap { it.value.chunkIdsList.hexFromProto() }
             val apkChunkIds = snapshot.appsMap.flatMap {
                 it.value.apk.splitsList.flatMap { split -> split.chunkIdsList.hexFromProto() }
             }
             appChunkIds.forEach { chunkId ->
-                appBlobs[chunkId] = snapshot.blobsMap[chunkId] ?: error("No Blob for chunkId")
+                val blob = snapshot.blobsMap[chunkId] ?: error("No Blob for chunkId")
+                appBlobs[blob.id] = ChunkIdBlobPair(chunkId, blob)
             }
             apkChunkIds.forEach { chunkId ->
-                apkBlobs[chunkId] = snapshot.blobsMap[chunkId] ?: error("No Blob for chunkId")
+                val blob = snapshot.blobsMap[chunkId] ?: error("No Blob for chunkId")
+                apkBlobs[blob.id] = ChunkIdBlobPair(chunkId, blob)
             }
         }
         // calculate sizes
-        val appSize = appBlobs.values.sumOf { it.length.toLong() }
-        val apkSize = apkBlobs.values.sumOf { it.length.toLong() }
+        val appSize = appBlobs.values.sumOf { it.blob.length.toLong() }
+        val apkSize = apkBlobs.values.sumOf { it.blob.length.toLong() }
         // let's assume it is unlikely that app data and APKs have blobs in common
         val totalSize = appSize + apkSize
         log.info { "Got ${appBlobs.size + apkBlobs.size} blobs worth $totalSize bytes to check." }
@@ -182,23 +190,21 @@ internal class Checker(
         val appTargetSize = min((targetSize * 0.75).roundToLong(), appSize) // 75% of targetSize
         log.info { "Sampling $targetSize bytes of which $appTargetSize bytes for apps." }
 
-        val blobSample = mutableMapOf<String, Blob>()
+        val blobSample = mutableListOf<ChunkIdBlobPair>()
         var currentSize = 0L
         // check apps first until we reach their target size
-        val appIterator = appBlobs.keys.shuffled().iterator() // random app blob iterator
+        val appIterator = appBlobs.values.shuffled().iterator() // random app blob iterator
         while (currentSize < appTargetSize && appIterator.hasNext()) {
-            val randomChunkId = appIterator.next()
-            val blob = appBlobs[randomChunkId] ?: error("No blob")
-            blobSample[randomChunkId] = blob
-            currentSize += blob.length
+            val pair = appIterator.next()
+            blobSample.add(pair)
+            currentSize += pair.blob.length
         }
         // now check APKs until we reach total targetSize
-        val apkIterator = apkBlobs.keys.shuffled().iterator() // random APK blob iterator
+        val apkIterator = apkBlobs.values.shuffled().iterator() // random APK blob iterator
         while (currentSize < targetSize && apkIterator.hasNext()) {
-            val randomChunkId = apkIterator.next()
-            val blob = apkBlobs[randomChunkId] ?: error("No blob")
-            blobSample[randomChunkId] = blob
-            currentSize += blob.length
+            val pair = apkIterator.next()
+            blobSample.add(pair)
+            currentSize += pair.blob.length
         }
         return blobSample
     }
@@ -216,3 +222,9 @@ internal class Checker(
         if (readChunkId != chunkId) throw GeneralSecurityException("ChunkId doesn't match")
     }
 }
+
+data class ChunkIdBlobPair(val chunkId: String, val blob: Blob) : Comparable<ChunkIdBlobPair> {
+    override fun compareTo(other: ChunkIdBlobPair): Int {
+        return chunkId.compareTo(other.chunkId)
+    }
+}
diff --git a/app/src/main/java/com/stevesoltys/seedvault/repo/CheckerResult.kt b/app/src/main/java/com/stevesoltys/seedvault/repo/CheckerResult.kt
index a2d5ecdf..2be2066d 100644
--- a/app/src/main/java/com/stevesoltys/seedvault/repo/CheckerResult.kt
+++ b/app/src/main/java/com/stevesoltys/seedvault/repo/CheckerResult.kt
@@ -24,7 +24,7 @@ sealed class CheckerResult {
         /**
          * The list of chunkIDs that had errors.
          */
-        val errorChunkIds: Set<String>,
+        val errorChunkIdBlobPairs: Set<ChunkIdBlobPair>,
     ) : CheckerResult() {
         val goodSnapshots: List<Snapshot>
         val badSnapshots: List<Snapshot>
@@ -32,9 +32,23 @@ sealed class CheckerResult {
         init {
             val good = mutableListOf<Snapshot>()
             val bad = mutableListOf<Snapshot>()
+            val errorChunkIds = errorChunkIdBlobPairs.map { it.chunkId }.toSet()
             snapshots.forEach { snapshot ->
-                val isGood = snapshot.blobsMap.keys.intersect(errorChunkIds).isEmpty()
-                if (isGood) good.add(snapshot) else bad.add(snapshot)
+                val badChunkIds = snapshot.blobsMap.keys.intersect(errorChunkIds)
+                if (badChunkIds.isEmpty()) {
+                    // snapshot doesn't contain chunks with erroneous blobs
+                    good.add(snapshot)
+                } else {
+                    // snapshot may contain chunks with erroneous blobs, check deeper
+                    val isBad = badChunkIds.any { chunkId ->
+                        val blob = snapshot.blobsMap[chunkId] ?: error("No blob for chunkId")
+                        // is this chunkId/blob pair in errorChunkIdBlobPairs?
+                        errorChunkIdBlobPairs.any { pair ->
+                            pair.chunkId == chunkId && pair.blob == blob
+                        }
+                    }
+                    if (isBad) bad.add(snapshot) else good.add(snapshot)
+                }
             }
             goodSnapshots = good
             badSnapshots = bad
diff --git a/app/src/test/java/com/stevesoltys/seedvault/repo/CheckerTest.kt b/app/src/test/java/com/stevesoltys/seedvault/repo/CheckerTest.kt
index 75241f40..cbfca6b1 100644
--- a/app/src/test/java/com/stevesoltys/seedvault/repo/CheckerTest.kt
+++ b/app/src/test/java/com/stevesoltys/seedvault/repo/CheckerTest.kt
@@ -82,6 +82,36 @@ internal class CheckerTest : TransportTest() {
         assertEquals(expectedSize, checker.getBackupSize())
     }
 
+    @Test
+    fun `getBackupSize returns size without under-counting blobs with same chunkId`() =
+        runBlocking {
+            val apk = apk.copy {
+                splits.clear()
+                splits.add(baseSplit.copy {
+                    this.chunkIds.clear()
+                    chunkIds.add(ByteString.fromHex(chunkId1))
+                })
+            }
+            val snapshot = snapshot.copy {
+                apps[packageName] = app.copy { this.apk = apk }
+                blobs.clear()
+            }
+            val snapshotMap = mapOf(
+                snapshotHandle1 to snapshot.copy {
+                    token = 1
+                    blobs[chunkId1] = blob1
+                },
+                snapshotHandle2 to snapshot.copy {
+                    token = 2
+                    blobs[chunkId1] = blob2
+                },
+            )
+            val expectedSize = blob1.length.toLong() + blob2.length.toLong()
+            expectLoadingSnapshots(snapshotMap)
+
+            assertEquals(expectedSize, checker.getBackupSize())
+        }
+
     @Test
     fun `check works even with no backup data`() = runBlocking {
         expectLoadingSnapshots(emptyMap())
@@ -138,7 +168,8 @@ internal class CheckerTest : TransportTest() {
         assertEquals(snapshotMap.values.toSet(), result.badSnapshots.toSet())
         assertEquals(emptyList<Snapshot>(), result.goodSnapshots)
         assertEquals(snapshotMap.size, result.existingSnapshots)
-        assertEquals(setOf(chunkId1, chunkId2), result.errorChunkIds)
+        val errorPairs = setOf(ChunkIdBlobPair(chunkId1, blob1), ChunkIdBlobPair(chunkId2, blob2))
+        assertEquals(errorPairs, result.errorChunkIdBlobPairs)
     }
 
     @Test
@@ -189,7 +220,8 @@ internal class CheckerTest : TransportTest() {
         assertEquals(listOf(snapshotMap[snapshotHandle1]), result.goodSnapshots)
         assertEquals(listOf(snapshotMap[snapshotHandle2]), result.badSnapshots)
         assertEquals(snapshotMap.size, result.existingSnapshots)
-        assertEquals(setOf(chunkId2), result.errorChunkIds)
+        val errorPairs = setOf(ChunkIdBlobPair(chunkId2, blob2))
+        assertEquals(errorPairs, result.errorChunkIdBlobPairs)
     }
 
     @Test
@@ -247,8 +279,8 @@ internal class CheckerTest : TransportTest() {
     fun `check prefers app data over APKs`() = runBlocking {
         val appDataBlob = blob {
             id = ByteString.copyFrom(Random.nextBytes(32))
-            length = Random.nextInt(0, Int.MAX_VALUE)
-            uncompressedLength = Random.nextInt(0, Int.MAX_VALUE)
+            length = Random.nextInt(1, Int.MAX_VALUE)
+            uncompressedLength = Random.nextInt(1, Int.MAX_VALUE)
         }
         val appDataBlobHandle1 = AppBackupFileType.Blob(repoId, appDataBlob.id.hexFromProto())
         val appDataChunkId = Random.nextBytes(32).toHexString()
@@ -266,6 +298,7 @@ internal class CheckerTest : TransportTest() {
         // only loading app data, not other blobs
         coEvery { loader.loadFile(appDataBlobHandle1, null) } throws SecurityException()
 
+        println("appDataBlob.length = $appDataBlob.length")
         every { nm.onCheckFinishedWithError(appDataBlob.length.toLong(), any()) } just Runs
 
         assertNull(checker.checkerResult)
@@ -275,7 +308,8 @@ internal class CheckerTest : TransportTest() {
         assertEquals(snapshotMap.values.toSet(), result.snapshots.toSet())
         assertEquals(snapshotMap.values.toSet(), result.badSnapshots.toSet())
         assertEquals(snapshotMap.size, result.existingSnapshots)
-        assertEquals(setOf(appDataChunkId), result.errorChunkIds)
+        val errorPairs = setOf(ChunkIdBlobPair(appDataChunkId, appDataBlob))
+        assertEquals(errorPairs, result.errorChunkIdBlobPairs)
 
         coVerify(exactly = 0) {
             loader.loadFile(blobHandle1, null)
@@ -283,6 +317,55 @@ internal class CheckerTest : TransportTest() {
         }
     }
 
+    @Test
+    fun `check doesn't skip broken blobs that have a fix with same chunkID`() = runBlocking {
+        // get "real" data for blob2
+        val messageDigest = MessageDigest.getInstance("SHA-256")
+        val data1 = getRandomByteArray() // broken blob
+        val data2 = getRandomByteArray() // data2 matches chunkId
+        val chunkId = messageDigest.digest(data2).toHexString()
+        val apk = apk.copy {
+            splits.clear()
+            splits.add(baseSplit.copy {
+                this.chunkIds.clear()
+                chunkIds.add(ByteString.fromHex(chunkId))
+            })
+        }
+        val snapshot = snapshot.copy {
+            apps[packageName] = app.copy { this.apk = apk }
+            blobs.clear()
+        }
+        val snapshotMap = mapOf(
+            snapshotHandle1 to snapshot.copy {
+                token = 1
+                blobs[chunkId] = blob1 // snapshot1 has broken blob for chunkId
+            },
+            snapshotHandle2 to snapshot.copy {
+                token = 2
+                blobs[chunkId] = blob2 // snapshot2 has fixed blob for chunkId
+            },
+        )
+
+        expectLoadingSnapshots(snapshotMap)
+        every { backendManager.requiresNetwork } returns Random.nextBoolean()
+
+        coEvery { loader.loadFile(blobHandle1, null) } returns ByteArrayInputStream(data1)
+        coEvery { loader.loadFile(blobHandle2, null) } returns ByteArrayInputStream(data2)
+
+        every { nm.onCheckFinishedWithError(any(), any()) } just Runs
+
+        assertNull(checker.checkerResult)
+        checker.check(100)
+        assertInstanceOf(CheckerResult.Error::class.java, checker.checkerResult)
+        val result = checker.checkerResult as CheckerResult.Error
+        assertEquals(snapshotMap.values.toSet(), result.snapshots.toSet())
+        assertEquals(setOf(snapshotMap[snapshotHandle2]), result.goodSnapshots.toSet())
+        assertEquals(setOf(snapshotMap[snapshotHandle1]), result.badSnapshots.toSet())
+        assertEquals(snapshotMap.size, result.existingSnapshots)
+        val errorPairs = setOf(ChunkIdBlobPair(chunkId, blob1))
+        assertEquals(errorPairs, result.errorChunkIdBlobPairs)
+    }
+
     private suspend fun expectLoadingSnapshots(
         snapshots: Map<AppBackupFileType.Snapshot, Snapshot>,
     ) {