Detect Duplicates In User Form Text Boxes

I have a user form with 4 text boxes and a command button. I have no problem looping through the textboxes to retrieve the value, but what is the best way to detect if any duplicates exist between the four text boxes? For example, in the command button's

If textbox one's value is "test" and textbox four's value is also "test", it should throw an exception. If a question like this has already been answered, a link to that post would surely suffice.