diff --git a/changelog/unreleased/issue-21820 b/changelog/unreleased/issue-21820
new file mode 100644
index 000000000..0fe079be1
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+++ b/changelog/unreleased/issue-21820
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+Bugfix: Correct handling of duplicate index entries
+
+Before restic 0.10.0, a bug could in very rare cases split information
+about a pack file across multiple index files. Since restic 0.17.0, any
+operation that rewrites the index (like `prune` or `repair packs`)
+could lose part of that information, resulting in errors in later
+`check` or `prune` runs. Those can be fixed by running `repair packs`.
+Note that only repositories using repository format version 1 might be affected.
+
+Split pack index entries are no longer lost during index rewrites. The
+`check` command now reports these cases as errors that can instead be fixed using
+the `repair packs` command. On older restic versions, running `repair index`
+twice also fixes the problem.
+
+https://github.com/restic/restic/issues/21820
+https://github.com/restic/restic/pull/21828