Rejecting duplicate inputs for two correct answers

I’m making a review for students solving factored equations and there are two correct x= values. I am having trouble rejecting entering a correct input twice. I’d like to post a message saying they’ve already used that answer and unsubmit one of the duplicates (if it is a correct answer). This work is on Slide 5.

My sample code

Maybe this recent thread will give you some ideas.

Thank you for the quick reply. I’ve not worked with table inputs yet (very new to CL). I’ll look for some sample code. Can you or anyone point me to a sample of check correct in a table (one that checks AND shows the student “correct” or “not correct”)?