duplicate transactions appear mysteriously

I've had this issue for at least a year or longer. I use Quicken for transactions, save/back up my file and when I open it the next time, a duplicate transaction mysteriously appears. It is random, could be with any category, and could be with any account. I can delete the duplicate transaction, but that also deletes the original transaction. Then I run the File Operations --> Validate and Repair, and I'm told there's an issue with a damaged category and to go look. I can't see a damaged category as I'm not sure what I'm looking for. I've had to turn off all online sync because syncing online just makes the duplicates worse. I end up running the Validate and Repair almost every time I use Quicken. Is there a way to fix this without starting from scratch? I've used this same file since 2013. I've done all the updating recommended in Quicken help files and this doesn't help. Thanks for any help.