How do show data with duplicates

Hi all,

I'm looking at some registration data and the question I'm trying to answer is: How many people sign up with their families or significant other?

I've filtered my data by "last name" and "unique IDs" assuming 2 things - if their unique ID is 1 +/- from each other and/or have same last names, they've signed up together.

Is there formula or function within Tableau so it will only show:

1) Unique ID that have duplicated last names &

2) Unique IDs that are 1 +/- from each other