Are you currently staring at an exact duplicate of yourself? Run. Run away. Read this page, then go back.
A common Doppleganger is what you are dealing with. Their bodies change shape, their minds sharpen, and their clothes morph around until eventually they look and think exactly like you. This is a major problem for most magic type adventurers because they know every spell that you do. Fortunately if you know a teleport spell you can just zip right by him and take off before he can do anything. Brawler type adventurers should have a little trouble with them, as they match your strength, but the gear they carry is just cheap knockoffs of your enchanted gear. Thief types should be able to sneak right by them, no problemo. On the event that you do get caught, just kick him in the crotch. Dopplegangers are used to their victims freaking out long enough for them to kill them quickly and cleanly, not jumping up and immediately disabling them with a quick popcorn maneuver. In fact, disregard most of this entire paragraph. No matter who you are, just kick a Doppleganger in the balls.
"Doppleghandis", as they are called, are an offshoot of the doppleganger species which by some odd branch in evolution copies its target completely down to every last molecule on their armor, ether particle in their magic swords, every synapse in their brain, leaving behind nothing of the original doppleghandi. A perfect duplicate of yourself that won't try to kill you. This, dear adventurer, is what we in the industry call a "One Up". You might call them "Free Men" or "Extra Life", but what it boils down to is that you or your doppleganger, NO fucking telling who the original was at this point (don't even try to argue with him), is now totally expendable. This may sound weird to you right now, and you may already be having an identity crisis, but worry not, dear adventurer. When it actually happens to you, you will realize a whole new world of possibilities opens up.
You can rock paper scissors each other and send the loser (or winner) home to your family while the winner (or loser) can (or has to) continue his adventuring career without any lingering regrets of abandoning your wife and children. You can charge into battle side by side, slaying dragons way way out of your league, knowing that two of you is twice the adventure. And twice the adventure brings twice the loot, twice the babes, and twice the potions if you're in to that sort of thing. You could keep him at the inn at all times to guard your valuables while you obtain more valuables. You could keep him in some town on standby waiting for the inevitable letter or newspaper article announcing your heroic, untimely, hilarious, tragic, or much deserved death so he can pick up exactly where you left off. Or you could just fight to a bloody end, winner now has two battleaxes forged by the last swing of the dying barbarian king's blacksmith hammer. How cool is that?
There is a third variation of the doppleganger, known as the "Koppelganger", It's a clone of Ted Koppel and was brought into existance when a curse that makes bad puns real things kicked in to effect in our own offices. do everyone a favor and kill it.