Unfortunately only one person can claim a dependent. You would need to remove this person from your return in order to e-file right now. If you still wish to claim this dependent you can file by mail; the IRS will follow up with you eventually to determine who is the rightful claimant. See this page for more information: https://ttlc.intuit.com/questions/1487992-how-do-i-find-out-if-someone-has-used-my-dependent-on-thei...
