Discover playlists could surface the same track twice with the
duplicates landing back-to-back — a "first song plays, then plays
again, skip works" symptom user reported on v2026.05.13.0. Root
cause: interleaveBuckets rotates one track per pass per bucket but
never tracks which IDs it has already emitted, so a track that's
both a dormant-artist pick AND a random-unheard pick comes out
once from each bucket.
On a single-user server the crossUser bucket is empty, so the
redistribute step rolls its slots into dormant + random. Their
output then interleaves d0, r0, d1, r1, … — and when d0 == r0
(common: a dormant-artist track is also valid for random-unheard)
the result is [X, X, …] with adjacent duplicates.
Fix: track seen track IDs across all buckets while interleaving;
skip already-taken IDs and advance to the next index in that
bucket. Dedup priority is bucket order, so a track in both
dormant and random comes from dormant.
Regression test covers the single-user case directly. The existing
round-robin test still passes — no shared IDs in that fixture.
Note: stale duplicates already written to drift / served as cached
playlists will clear naturally on the next playlist rebuild (the
03:00-local refresh, or any manual /api/me/playlists/refresh).
bvandeusen
c29d25d1cb