Probably the simplest method is to use a Countif formula.

If your list of EID numbers are in column A, put the following in B1 (or other suitable spot) and copy it down. The dollar signs are important so the range you are looking at doesn't shift as you copy it down.

=COUNTIF($A$1:$A$15,A1)

This particular method will show a "1" for unique numbers, and each duplicate will show "2" or higher depending how many duplicates there are.