by options select Merge Account Option, Then by the 1st tab "Search" enter the criteria that matches both accounts then you can click on Merge, the fist account you click merge will be the main account.
There's also a screen where you can find all possible duplicates, Entries > Duplicates, click on show duplicates, if you want to check just on a group of accounts you can query in the report screen and click use report selection and then click "Show Duplicates"
