If you import your transactions (for example, from a CSV file from your bank statement), they are checked for duplications and will be flagged up. However, entering them manually won’t be checked. Maybe your best bet would be to try and reconcile your accounts to see where there are any errors to correct them easier.
There’s an article here that shows you how to do this:
http://community.quickfile.co.uk/t/reconciling-your-bank/8844
I hope that helps!