Help comparing two columns in excel, returning third w/dups

Registration date
Tuesday March 7, 2017
Last seen
March 7, 2017

I have have found on here some help in comparing two columns and if there is a match, copying a third column, but I need a way to receive an error code if there is a duplicate rather than just return the first occurrence. I want to find a district for each of the zip codes below. I have a list of all zips and their corresponding distinct, but some zips overlap and are covered by more than one district. I'd like the district for each zip in Column A to be listed next to in in Column B - however, 22498 should return an error since it has two districts. See an example below. Thanks!

A (sites) B C (source data - zips) D (source data - district)
35578 - 32142 District 1
35579 - 22498 District 2
35501 - 22498 District 3
20163 - 32142 District 1
22498 - 35579 District 4
32142 -