Q. Will I get in trouble legally?
A. Somebody will. Maybe you, maybe them.
If someone else claimed you, as a dependent, inappropriately, and if they file first, your return will be rejected if e-filed. You would then need to file a return on paper, claiming yourself, if appropriate. The IRS will process your return and send you your refund, in the normal time. Shortly (up to a year) thereafter, you’ll receive a letter from the IRS, stating that your exemption was claimed on another return. It will tell you that if you made a mistake to file an amended return and if you didn't make a mistake to do nothing. The other party will get the same letter you did. If one of you doesn't file an amended return, unclaiming the exemption, the next letter, from the IRS, will require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits; loser gets to pay the IRS back with penalties and interest.
If your e-file was not rejected, then theirs was. That shoulda/woulda been a red flag to them. They needed to file a paper return to claim you. So, there's a chance that they didn't actually claim you.
