The collage builder pulled exactly 4 tracks (in playlist position
order) and rendered each track's album cover into a 2x2 cell. A
playlist where the first 4 tracks share an album rendered as four
identical cells of the same cover; a "Songs like X" mix where two
similar artists dominated produced very similar collages.
Now pulls 50 rows from the same query and drops adjacent /
duplicate covers in Go before passing to the renderer. NULL and
empty cover_art_paths are NOT deduped (they each render the
fallback glyph in their own cell — the right behavior for a
partially-covered playlist).
Preserves playlist position order: the first occurrence of each
unique cover wins. Five unit tests cover the behavior: drops
duplicates, keeps NULLs, handles empty-string paths, preserves
order, mixed null/dup case.
Tangential to the For-You CTA slice but lands here as a small
quality-of-life fix that benefits every playlist's collage.
