In the shard-local implementation of create collection we take the MODE_IX lock on the collection. This means that multiple create operations can run concurrently for the same namespace. Consider the following problematic scenario:

1. operation_A: starts with OFCV 8.0 sees that the collection does not exists yet
2. FCV transition from 8.0 to 9.0
3. operation_B: starts with OFCV 9.0 sees that the collection does not exists yet
4. operation_A (OFCV 8.0): create the legacy timeseries buckets collection (`db.system.buckets.ts_coll`)
5. operation_B (OFCV 9.0): create a viewless timeseries (`db.ts_coll`)

So we end up with an inconsistent state in the catalog where both `db.ts_coll` and `db.system.buckets.ts_coll` exists as timeseries collections.

The bug can occur only if:

• viewless timeseries feature flag is enabled in FCV 9.0
• We are on a replicaset cluster. On sharded clusters create operations are serialized through the DDL collection lock.