profile_aggregates=> create index concurrently foo_idx on agg (status, foots, created_ts);
^CCancel request sent ERROR: canceling statement due to user request profile_aggregates=> profile_aggregates=> create index concurrently foo_idx on agg (status, foots, created_ts); ERROR: relation " foo_idx" already exists
Are these operations not atomic ?
No, being atomic would interfere with doing things concurrently. Per the docs:
In a concurrent index build, the index is actually entered as an “invalid” index into the system catalogs in one transaction, then two table scans occur in two more transactions.
...
If a problem arises while scanning the table, such as a deadlock or a uniqueness violation in a unique index, the CREATE INDEX command will fail but leave behind an “invalid” index.
"Problem" includes you forcibly killing it while it is running.