Score adds up when I don.t want it to!

I think the solution to this is easy but for some reason I just can't figure it out.I have a simple 5-question test. I created a variable (testscore) that starts out at "0". For every correct answer, I add 20 to the variable. If the learner clicks the back button and answers the question again, it adds another 20 to the score.Basically, I want the score to stop adding if the question has already been answered once. WWYD?

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