I have imported a large(ish) number of cutomer records from Salesforce and their contacts. I did not do it in one import, rather I imported the customers and then matched the customers with the contacts using a vlookup on their Salesforce id.  I was unaware that vlookup is not case sensitive and that Saleforce id's are. This lead to a large number of contacts in the wrong company.

Now that I know, I can do another import with the right company, but I don't want these contacts to have two companies with one being wrong. If I do an update type import of contacts with the Company field mapped to the correct customer (internal id) will it overwrite the company that the contact is attached to or will it add a new relationship? If the latter, is there any way I can avoid a new relationship and force an overwrite?