A Spawnable actor gets "SequencerActor" tag on spawn, but this tag addition does not seem to check already assigned tag(s). So the actor can receive SequencerActor tags multiple times.
1. Open the attached project
2. Open the level sequencer
3. Click on the actor(spawnable in the sequencer) and check the actor's tag
4. Scrub the sequencer timeline and make the actor despawn and respawn, or simply uncheck/check Spawn
5. Observe the actor now has even more SequencerActor tags
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40665 in the post.