PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> CREATE TABLE test(id uuid PRIMARY KEY);
> ALTER TABLE test ADD IF NOT EXISTS id uuid PRIMARY KEY;
> Output: `NOTICE: column "id" of relation "test" already exists, skipping
> ERROR: multiple primary keys for table "test" are not allowed`
Yeah, this is a well-known issue: the IF NOT EXISTS only conditionalizes
creation of the column, but it's reasonable to expect that it should
conditionalize creation of the index as well. There's work afoot to
improve this [1], but it's not moving very fast; at the earliest you
might see it fixed in v13.
regards, tom lane
[1] https://www.postgresql.org/message-id/flat/10365.1558909428%40sss.pgh.pa.us