fix: feed repitition

internal/repository/post_repo.go

@@ -219,14 +219,27 @@ func (r *PostRepo) ListByUser(ctx context.Context, userID string, limit, offset
 //	tier 0 — posts by people the viewer follows that they haven't liked yet
 //	tier 1 — everything else (followed-and-liked, followers', strangers')
 //
-// The viewer's own posts and anything involving a block (either direction)
-// are excluded. The tier is computed in ORDER BY via correlated EXISTS, so no
-// extra columns leak into scanPost. No total is computed (infinite scroll
-// infers "more" from a full page); offset paging keeps it simple. At larger
-// scale this would move to a precomputed fan-out / keyset cursor.
-func (r *PostRepo) FeedForViewer(ctx context.Context, viewerID string, limit, offset int) ([]model.Post, error) {
+// The viewer's own posts, anything involving a block (either direction), and
+// any id in `exclude` (the already-served set) are filtered out. The tier is
+// computed in ORDER BY via correlated EXISTS, so no extra columns leak into
+// scanPost. No total is computed (infinite scroll infers "more" from a full
+// page).
+//
+// With a non-empty `exclude` the caller drives pagination purely by exclusion
+// (pass offset 0): since the tier depends on viewer state — liking a post
+// moves it between tiers — an OFFSET would drift and repeat/skip rows, so the
+// seen-set is the correct cursor. Without Redis, `exclude` is empty and OFFSET
+// paginates (the frontend de-dups the rare drift). At larger scale this moves
+// to a precomputed fan-out / keyset cursor.
+func (r *PostRepo) FeedForViewer(ctx context.Context, viewerID string, limit, offset int, exclude []string) ([]model.Post, error) {
+	// nil would encode as SQL NULL, and `id <> ALL(NULL)` is NULL (excludes
+	// every row); a non-nil empty array means "exclude nothing".
+	if exclude == nil {
+		exclude = []string{}
+	}
 	q := `SELECT ` + postSelectColumns + postSelectFrom + `
 		WHERE p.user_id <> $1
+		  AND p.id <> ALL($4)
 		  AND NOT EXISTS (
 		      SELECT 1 FROM user_blocks b
 		      WHERE (b.blocker_id = $1 AND b.blocked_id = p.user_id)
@@ -247,7 +260,7 @@ func (r *PostRepo) FeedForViewer(ctx context.Context, viewerID string, limit, of
 		   END) ASC,
 		  p.created_at DESC, p.id DESC
 		LIMIT $2 OFFSET $3`
-	posts, _, err := r.collectPosts(ctx, 0, q, viewerID, limit, offset)
+	posts, _, err := r.collectPosts(ctx, 0, q, viewerID, limit, offset, exclude)
 	return posts, err
 }