Stamp it!
During implementation, I do see duplicated transactions being recorded in the accounting software. Most of the accounting software will prompt user whenever you try to record a duplicated transaction; either duplicated invoice, bill or the cheque number. However, when you are “busy” or try to “multi-task”, you may simply slip your mind and ignore the warning message.
To prevent mistakes made in recording transactions, I prefer using an old school method — rubber stamp.
After recording a transaction, I will stamp on the physical document. I bought a rubber stamp with the word “ENTERED” and a date beside it. It is useful, I can differentiate the entered document easily.
Stamping the document is an old school method, which had been used for decades, but it is an effective method. A method which works for most bookkeeper.