B-Tree deduplication can also help to control duplicate index tuples caused by MVCC "version churn" -- including in unique indexes. See the paragraph that begin

B-Tree deduplication can also help to control duplicate index tuples caused by MVCC "version churn" -- including in unique indexes. See the paragraph that begins with "B-Tree indexes are not directly aware that under MVCC..." here: https://www.postgresql.org/docs/devel/btree-implementation.html#BTREE-DEDUPLICATION https://www.postgresql.org/docs/devel/btree-implementation.h...