Warning Dialogue on form submissions

Just like a validation rule, can we have a “warning” rule which prompts the user to double check the data before submitting?

For example, we have users that put time sheets in, but sometimes they have to put 2x timesheets for the same day. I would like to be able to pop up a warning that works like:
IF day = not unique, then warn user “Day already exists, are you sure you want to enter times in again?”

Would this be hard to do?


Were you ever able to figure this out @Bevcom?