Is there a way to check for duplicate entries in an account register?

There's no tool that you can run where the program will run through all your transactions and present a list of transactions that might be duplicate transactions.  Within Registers you can use the "Search" box to put part or all of a payee's name in, allowing you to scroll through transactions looking for dups, and Spending by payee reports would provide the same functionality if the possible duplication spanned Accounts.

You can sort the register (or all Transactions) by any of the column headings. Like the payee or amount.  Then look through the list.