> The big difference is that duplicate index entries can still occur in B-tree indexes. GIN indexes have to integrate new entries into the posting list every time, which makes data modifications less efficient (this is mitigated by the pending list). In contrast, PostgreSQL deduplicates B-tree entries only when it would otherwise have to split the index page. So it has to do the extra work only occasionally, and only when it would have to do extra work anyway. This extra work is balanced by the reduced need for page splits and the resulting smaller index size.

https://www.cybertec-postgresql.com/en/b-tree-index-deduplic...