Have you noticed how many Discworld Stamps have doppelgangers in the Roundworld? I suppose that once Moist, aided and abetted by Teemer & Spools, accidentally replicated the Penny Black to a large extent with the Penny Patrician the postal authorities over here were bound to play a game of tit-for-tat and base some of their designs on Discworld stamps.
"Wait", I hear you say, "The Roundworld examples date back sometimes more than a century before the DW examples appeared here. " What you are forgetting is that the two universes are travelling down different trouser legs of time; and these trouser legs are not necessarily a matching pair, with one stretched to a slimmer fit, while the other is all baggy and crumpled,  with huge turn-ups. This results in time in the parallel universes not necessarily matching at the knees or anywhere else, and may run alternatively faster and slower in one leg compared to the other, or even backwards.

Here is a list of the pairs of stamps which have been identified. It is not complete by any means. Some pairs show marked similarities, while for others only some elements of one may have inspired some detail on the other.

The Stamps The Dopplegangers The Penny Patrician The 1840 Penny black Year of the Prawn 2p 1889 Reichspost Year of the Prawn 10p Morporkia Natal 1902 high value definitive Assassins Guild Three Pence De la Rue trial stamp The 2005 Merchants Guild 1939 Barbados commemorative Twoshirts 2p Cabbage Macerating 1915 Caroline Islands German colonial Zlobenian 1 Zloty Imperial Russia 20 Kopek Sto Lat 10p Jaipur Silver Jubilee 1947 Anoia 1p special issue Prince of Wales Hospital Fund 1897 Klatch 10 Wol Iraq 1930 6 Anna Djelibeybi 1 Talon Egypt 1000 year anniversary of Al-Azhar University. (I am not too convinced on this example) Tsort 1 Drachma Aden states 1942 (I am clutching at straws with this one) Ankh-Morpork postage dues Great Britain pre-decimal postage dues Initial Patrician and 5th Anniversary Patrician Queen Victoria high value 1865-83 Sto Lat 2p Cape of Good Hope 1897 Year of Pensive Hare 2p Antigua George V 1d $2 Merchants Guild Civic GB George V high values Ankh-Morpork Great Inventors Canada 1927 special issue $1 International Parcel Post 1936 Tannu Tuva 1R diamond Soul Cake Duck Penny Tristan da Cunha 1948 One Penny     There are probably more examples that could be added to this page, especially some more recent ones. If anybody has any such examples let me know on the Facebook page. Someone with inside information will know some.