Double Struck. Literally struck twice; an intentional double blow, as in a coining press designed to deliver two cycles of the press before the struck piece is ejected. The purpose of double striking is to provide a sharper, better strike with greater assurance that every cavity of the die is filled, the edge is complete and the rim/edge juncture is as perfect as possible. Some proof surface pieces, coins or medals, are often double struck to provide the higher quality piece that could not be done with production run coinage and production run dies. See multiple striking.

One might think if double struck is better than single struck, then why not three (or more) blows? The answer is that with each striking the piece becomes work hardened; a third (or more) blow would not move any more metal to fill the entire cavities of the dies. Production run coining must accomplish this in one blow. Proof coins and medal striking can afford the luxury of a second blow.

Double struck anomalies. Most double struck anomalies are from an unintentional second blow. Often there is some movement of the blank so the second blow is not exactly in the same position as the first. Thus the second image is superimposed on the first, but some evidence of the first image still exists.

Double struck can also be from a roller die on the edge of a numismatic piece; from a punch or logotype struck twice; or counterstamp with two images from the same letters or punches. In a sense, double struck may explain any act of impression which is performed twice.

Reference:

NE42 {1982} Doty, p 110-111.