Collectors of brockages know that all brockages are scarce. Brockages are usually not unique. If one was produced by accident, it is likely that a few more were struck. For any given type of World coin, nearly all Brockages will be found on the obverse. Foreign Mints usually place the Reverse Die in the upper hammer position.
Since Brockages are usually produced when the upper Coining Die is covered by a previously struck coin, the coin will show two images of the same design. A brockage error can only occur when there are two coins involved. One of the coins involved will always be a struck coin which has not ejected properly. That struck coin will find its way back between the dies and will be struck next to a blank planchet which was fed into the collar. The image of that first struck coin will be impressed into that side of the blank planchet. The result will be a second coin which has images of the first coin impressed into it. Those images will be pressed into the coin and the image will be in reverse. This incuse sunken image is known as a brockage.
It is difficult to find both obverse and reverse brockages of the same coin type. Here are some examples:
A Brockage Pair of English Shillings (Obverse and Reverse)