can I file the defendants answer to my summons and complaint?

He probably filed a duplicate original. If he did not, you will win by default, unless you file it for him.

Yes. You can file the answer. I have to do this ALL the time when I am dealing with pro se defendants.

No, this will not hurt your case.

EDIT: The other answerers are incorrect. You cannot obtain a default judgment under these circumstances because the defendant HAS answered. You can only obtain a default judgment when the defendant does not respond in any way, shape or form. If you wanted to be obnoxious about this, you could send him a Notice of Intent to Take a Default Judgment based on his failure to mail the answer to the court. But all he has to do is to mail the answer to the court and you don't get the default judgment. And judges do not like litigants who play default judgment games - especially when the defendant did try to submit a good faith answer.

Your instincts to file the answer yourself are right on target. In the alternative, you can mail the answer back to him (keep a copy for yourself) with a polite letter that reminds him that he needs to send the original to the court.

Disregard the 3 prior answers.

You may not, under any circumstances, file a court document other than your own. It is possible that you were sent a duplicate original instead of a photocopy, a mistake common among pro per litigants.

You did not state if the face page of the document you received was stamped "Filed on__" or has any other indication that it was filed in a timely manner. As such, you will have to check the case docket to see if the answer has been filed.

DO NOT send a letter of any kind to defendant, such an action could only irreparably harm your case.