how to filter for NON-unique entries in a view?

I have a form, where people can submit their support for some initiative.

I would like to create a view which shows entries that have the same name, surname AND Email to have the possibility to manually check for duplicates. Having unique values for fields is not an option, bacause sometimes there are people with the same name and sometimes families sign the form using only one mail-adress. So we want to check manually from time to time.

So I would need a filter option, that says: “is NOT unique”

My idea would be:

Name – is NOT unique – [get param=”Name”] AND

Surname – is NOT unique – [get param=”Surname”] AND

Mail – is NOT unique – [get param=”Mail”]

How can I do that?