You've probably got two sync paths feeding the same session.
Common causes:
A Garmin watch syncing to Apple Health, with Garmin also connected directly to CrewLAB
A COROS device syncing to Apple Health, with COROS also connected directly
The fix
Open Connected Apps and remove the redundant integration. Keep one path per device. Workouts continue syncing through whichever you leave.
If your device already syncs to Apple Health, Apple Health to CrewLAB is usually simplest.