First, confirm the camera is still set to Backup in the card/slot setup. If that changed, playback and delete behavior can seem inconsistent.

Also check the Playback Folder setting. If playback is set to show images in a different way than before, the camera may be displaying files from both cards separately, which would make every shot appear twice.

If the settings look correct, try reformatting both memory cards in the camera after backing up any needed files. A card/file structure issue can sometimes cause odd playback behavior.

If that still doesn’t fix it, do a two-button reset and then recheck the card and playback settings. If you have custom settings saved, back them up first so you can restore them afterward.

In short: check slot function = Backup, verify Playback Folder, then reformat cards, and finally try a reset if needed.