It may have been caused by:
- Reviewing some cards on one device, then
- Modifying the cards on another device (eg by rebuilding/emptying a filtered deck), and then
- Syncing the two devices
The change in 2 will overwrite the change in 1, so the cards end up getting shown again. When you review them again, you end up with duplicate log entries.
You can avoid this by always syncing at the start and end of each session: Syncing in Anki: Getting Started - YouTube