Printing on both sides occurs when an already printed sheet is accidentally, or more rarely deliberately, fed through the press again and the stamp design is printed on the reverse as well.

The impressions on the front and reverse usually do not register exactly with one another. The reverse image is a genuine second printing, so its lines and details have the same character as those of the normal impression. This distinguishes it from ink show-through, where the image on the reverse is merely caused by ink penetrating the paper and is therefore paler and more diffuse.

On perforated stamps, the two impressions usually occupy different positions in relation to the stamp margins and perforations because the sheet was not reinserted in exactly the same position for the second printing.