I have a list of 30,000 unique user ID's and the date when each joined the service. I have a separate list of about 8,000 unique user ID's as a subset of the larger list, which lists the dates when each user made payments. I need to find the average number of days from when a user signed up to their first payment date. The problem is that many of the 8,000 users on the second list are listed multiple times because they made multiple payments. See the attachment to get a better idea of what I'm talking about.

I have tried using conditional formatting to identify duplicate user ID values, but that didn't get me very far. Is there a function I could use to isolate all of the users that made payments and to delete all of the users that did not make payments? Any help would be very much appreciated.

Problem - Product Scenario 2013 (1).xlsx