What if the other parent has already claimed the kid on there taxes?

Answer 1

Then if you try to e-file, your e-file will be rejected.

Because of this, there is an urban legend that the first person to file gets to claim the exemption. This is not true.

If you think you are the one who is legally entitled to claim the deduction, then mail in a paper return claiming the deduction. NO ONE can help you or accept any complaints until after you have filed a return claiming the exemption. You will have to mail it in on paper, rather than e-filing. (You can still use a computer to help you prepare your taxes. But when you get to the filing step, tell it you want to file on paper.)

After you return is processed, the IRS will detect that two people have claimed the same kid. They will send letters to both of you asking if either of you wants to change your mind. If nobody backs down, they will send a form asking you to substantiate your right to claim the kid. They will look at your responses and then decide what to do next.