Find and append new unique values to secondary table

I have an app where I run a daily upload of data (much of it duplicate due to the nature of the source), but sometimes my daily upload includes a new record that needs to be added to a secondary table within the app. The key field in the secondary table matches one of the fields in my daily upload. I have set up a QB action that should identify new unique values in the field related to the secondary table's key and copy these records. But I am running into a problem. Sometimes my upload will include several rows with the same value in the field that I want to use to populate the secondary table's key field . When this happens, my action is registering an error. I only need the value in one of the duplicate lines to write to the secondary table, but I need to preserve all of the lines in the primary table. Any suggestions?