Unfortunately, we have a lot of dirty data in Apollo. Duplicate accounts are a bit of a problem at this point and it is one of our responsibilities to keep an eye out for them and merge them when we come across any.
On the account page, Apollo will alert you if there is a duplicate. Look at both account pages and determine which one has more engagement. Simply merge the Account with less engagement into the account that has more engagement.
This is done by navigating to the account that's been used primarily. On the top right-hand corner of the screen, you will see a message indicating that the account has a duplicate. Click on the hyper link titled "Click here to merge this duplicate with the current account"