What you need to do is create a template column of the two columns you need to “combine” to apply your rollup. Make a template of the user’s email (or other unique identifier) and the date (you may also want to “lock” in the date formatting by first creating a template column that stores the date as text first — this might not be necessary, but it’s still something I do in these cases).
Once you’ve got the template column (i.e. email@email.com April 1, 2021), create a multiple relation that check if the template matches itself. Then, create a rollup on this relation to get the sum of all the matches.