Question, you did remove the wires from the receptacle before testing for voltage right?
What Kok328 said about the bridge tab, did you understand that? On a new outlet, you have two screws on each side, two are silver and two are darker colored, and then you have a single one that is usually green. on the sides you have the sets of screws and in between the screws you have a metal tab called a bridge, i bridges a connection from one screw to the other on each set of screws. That way if you only have one wire coming in, you attach the black to one of the darker screws, the white to one of the silver screws, and the ground to the green. as long as the bridges are in place it doesnt matter what screw you attach to in the sets as long as you put a black on the barker set and the white to the silver set of screws. The outlets are separate of the other, and the bridge connects them so a single wire can power the two outlets.
What Paul said about the receptacle having a constant on and a switched plug all on the same receptacle is the most common reason you will have two hots in the same receptacle box.
Attached is a pic of a normal duplex outlet with the bridges intact. You would grab the tab with side cutters and break it out of place to separate the two plugs.