A common mistake people make in QBO is to forget to check all the unreviewed bank feeds. One thing many don't realize is if they delete a transaction that was either added or matched from the bank feeds is that it goes right back to the bank feeds review section. An easy mistake to make is that there might have been some that have auto add rules on them. There are times when you might be dealing with duplicates and you legitimately need to check the box on the bank feeds to the left of the transaction(s) to exclude them. But if you don't take care of the ones needing to be excluded right away they might end up being added right back to your register on accident or by the auto rules after you click the update button.
The best way to ensure you have no duplicates added is to reconcile the last month of the year. Look for any uncleared transactions that don't belong. If you are reconciling a credit card then you want to reconcile the statement date that spills into January to be safe you have covered all the December transactions.
It is wise to click the gear icon then click account and settings. From there click Advanced then click the "Close the Books" on Dec 31st after you are ready to prevent unwanted changes to your books for transactions dated on or before the close date.
