How to prevent students from taking an "assigned" Kahoot more than once. (aka 1 Student: 1 Test Grade)

The problem arises when I use Kahoot as a real exam grade remotely.  Many students will simply put in a dummy name, get all the answers, and then do the kahoot again under their real name, after they know all the answers, rendering the test useless.

Perhaps I could (or Kahoot could offer) enter a database of acceptable names (or even numbers) that represent each student before hand, so that I know who is who. It's all about the teacher controlling who is taking the test.  I have no control, so therefore, students "game the system".  Do you have any workarounds for this?

Kahoot is fun, but now that remote learning is here to stay, I wish I could give a real test on it.  Right now, test security is compromised because students can do a kahoot as many times beforehand with pseudonyms, and I have no idea who is cheating or not.