A sheet that is twice pulled and lifted never so little off the form after it was first pulled, does most commonly (through the play of the joints of the tympan) take a double impression: this sheet is said to double. Or if the pressman run in so, as the foreside of the platen print with the first pull into part of the second pull, or the hind edge of the platen print with his second pull into part of his first pull; either of these twice printing is called Doubling.
Doubling also happens through the loose hanging of the platen, and through too much play the tenons of the head may have in the mortises of the cheeks; and indeed through many wearings and crazinesses that often happen in several parts of the press. — M.
Moxon's account of doubling and its causes refer to the two pull wooden presses, but some of them apply equally to iron presses. In composing, the term is applied when a word, a line, a sentence, or any part of the copy is composed twice; that is, if the compositor's eye catch a part that he has already composed, and he repeats it.