Merging two lists, finding and keeping only duplicates

Hi. I have a problem I'm trying to solve. I am merging two lists. Both are lists of names. There are some people on both lists. I want to eliminate the records of the people who were only listed in one of the original two lists and be left only with the list of names who are common to both lists.

Each person on both lists has an ID number that I can use to compare. But while Excel seems to make it easy to eliminate duplicates, I want to do the opposite. I want to eliminate the records of people who are not on both lists and keep only the diplicate records in the spreadsheet.

The only way I've been able to figure out how to do this is to go through manually, mark a column for the duplicates, then sort on that column to collect all the duplicate names and delete the rest. This is very time consuming with a list that contains thousands of names.

Does anyone have a method to identify and segregate the duplicates in a spreadhseet so I can delete the rest?

Thanks.