A duplex canceller was a hand stamp used to cancel

postage stamp A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage (the cost involved in moving, insuring, or registering mail), who then affix the stamp to the fa ...

s and imprint a dated

postmark A postmark is a postal marking made on an envelope, parcel, postcard or the like, indicating the place, date and time that the item was delivered into the care of a postal service, or sometimes indicating where and when received or in transit. ...

applied simultaneously with the one device. The device had a steel die, generally circular, which printed the location of the cancel, together with the time and date of cancel. This die was held in place by a handle with an obliteration marker, often oval shaped, off to the right side that was applied over the postage stamp to prevent its reuse. The ink came from an ink pad. In many countries the obliterator part of the canceller was coded, in various ways, to identify the post office. In the United States, they were first used in the 1860s and use continued into the 1940s. Some machine cancelling devices like the French Daguin machine or the Italian Dani Machine also applied both the "killer" and the date stamp simultaneously. Especially in the Italian literature (and in German literature about Italian cancels) these cancelling devices are referred as duplex cancel.

References

{{reflist Postal markings Philatelic terminology