If you have a duplicate vendor in your system, and the duplicate has no payment history, you can simply delete it. To do this, go to Settings (gear icon) > Client Setup. Load the vendor and click "Delete" on the top-right. 

If the duplicate vendor has a payment history, you will not be able to delete it outright. Instead, you can merge the two vendors together. To do this, go to Settings (gear icon) > Transfer/Merge. Select "Vendor" in the Account Type dropdown. On the left hand-side, select the vendor you wish to remove. On the right-hand side, select the vendor you wish to retain. Click "Execute Merge" to merge the two together. The vendor on the left will be deleted and the pertaining information/accounting balance will be moved to the other vendor. Please Note: this action is irreversible!