There is nothing you can do to prevent the other parent from claiming the child ... so if your efile is rejected for this situation all you can do is mail in the return to assert your claim to the child. Then expect an IRS letter sometime in the next year asking for the person who made the mistake to amend their return. If you have any contact with the other parent you may want to discuss this and explain you have the right to claim the child so the other parent knows and everyone can avoid a mess later.
