Cannot submit tax return because it says Social Security number is being used for another file

Mail your return with an explanation that your efile was rejected because your social security number was already used. Monitor your credit cards And financial accounts for possible identity theft. 

There are a couple of possibilities here.  1) A parent or other family member has claimed you as a dependent; or 2) you are a victim of identity theft and the thief already filed his/her claim under your SSN.  It might take an in-person visit to your lRS office to get it straightened out

I would ask friends and family if they claimed you as a dependent and go from there.  Good luck.  The IRS will eventually get your refund to you if you were an ID theft victim.