created a customer search, but after adding a formula field it started duplicating the results. Any way to prevent?

One or more of the fields used in the formula must be calling a value which produces more than one result. This happens often if you introduce field names on "sub" or joined records, off of the Customer record (such as Sales Orders, Addresses, etc). You'll either need to restrict the criteria to not include multiple results per customer (by adjusting the criteria) OR you will need to group results together in a Summary. Under the "Results" tab, you could try adding a Summary Type of "Group" on the Customer ID. That will group the resulting records by the Customers ID, and give you unique summarized list. (You may need to add more summaries to introduce your other results values, like your Formula field.)

@Dan Curtis thanks! Grouping ending up working out.