Selecting Distinct Product

I know "select  distinct ProductId from Product  " selects  one product only  one time
 
how can i apply the same logic in a query like give bellow 
 
SELECT

    I.QuoteRequestItemId,

     I.ProductId

    ,P.StorePartNumber

    from

    QuoteRequestItem I  left join Product P
    on I.ProductId = P.ProductId
 
ie  product with  same productId  should be considered only once
there should not be more than one row  with  same productId