Hello Excel gurus

I'm running into a little challenge I need advice on.
I've got a set of 7 sheets (tabs) in one same xlsx.

Essentially its a list of names, emails etc. Each tab correspond to an event attendee list of people.
I'd like be able to easely compare data (currently each person in one row) and see when people have "duplicates", meaning they attended sevral events.
For info the sheets are not very long with possibly 30-50 entries for each.

What approach can I take for the task?
Is their a way to perform this by keeping the data in the seperate tabs or must it all first be brough into one and same tab?

Thank you !