If you have created these records from an import, make sure the record does not appear twice in your import file. If it does, you can either merge the records or delete one of them.
Likewise for if your issue is regarding a duplicate account, with one a Prospect and one a Customer. If you create the prospect within CRM and then the integration creates the customer, you will need to merge the two accounts by doing the following:
Using the list view, tick the tick box of both iterations of the account
Select the dropdown arrow next to the top tick box
Select Merge
Move the top primary box over which record you would like to be the primary record
Select Merge