While the object is selected, go to the duplicate options (by clicking the little square next to "Edit>Duplicate"). In that window click "Edit>Reset Settings".
Now, the next part depends on the direction on which you want to mirror. To mirror in the x-axis change the value in the first text box next to "scale" to -1. To mirror in the y-axis change the value in the second text box next to "scale" to -1. To mirror in the z-axis change the value in the third text box next to "scale" to -1.

I probab ly should have mentioned this at the begining, but first you have to make sure that the pivot is where you want it. I attached a picture showing an object being mirrored on the x axis. To move the pivot to that place chose the move tool, then press "insert" on your keyboard, then hold down "x" (to snap to the grid) and then move the pivot in the direction of the center and it should snap there.