Duplicate Vendors?
Many times vendors are entered into FE twice by accident. This is usually done because a user cannot find a vendor and then assumes that the vendor has never been entered into the system.
Fortunately this problem can be resolved easily and quickly. There is an option called Merge Vendors located in Administration. This will allow you to combine the two vendors into one, activity and all.
As you can see, there are two fields on this screen. So what's the difference? Well, Primary vendor is the record you would like to merge into or the record that will remain. Duplicate vendor is the record you would like to merge into the primary vendor or the record that will be removed.
So let's say we want to merge two vendor records.
Be sure you have a current backup. 
Begin by selecting the vendor records by clicking on the binoculars and browsing for the records. Then click Merge Now to combine the two records into one: the primary record. It's as simple as that!
Oh and by the way, this same process is available in Accounts Receivable for clients.