I used to get that message too. Basically, it means that you might have two or more tabs that have the same exercise open at once, and Khan Academy can't compute the different data being sent all at once. If you only have one tab for each exercise open at once, it should fix the problem. :)

Small problem, i dont have multiple tabs open on it, so what exactly does that mean, someone else is using my account?

I tried this and it worked thank you so much “leezero2002!”