A mirror of opposition (DMG 262) (92,000 gp; 45 lbs.) does pretty much exactly what you want:
This item resembles a normal mirror about 4 feet long and 3 feet wide. It can be hung or placed on a surface and then activated by speaking a command word. The same command word deactivates the mirror. If a creature sees its reflection in the mirror’s surface, an exact duplicate of that creature comes into being. This opposite immediately attacks the original. The duplicate has all the possessions and powers of its original (including magic). Upon the defeat or destruction of either the duplicate or the original, the duplicate and her items disappear completely. The mirror functions up to four times per day.
However, the mirror is, potentially, infinitely exploitable—be careful introducing it into your campaign, and adjudicate carefully that word defeat.