Stamps on repeat…
Postal administrations worldwide know that the first stamps issued by the country itself are eagerly collected. However, they also know that it is impossible for most collectors to obtain those stamps. Therefore, especially at international stamp exhibitions, stamps are often issued featuring a reproduction of the first stamp or stamps.
In the Netherlands, this happened for the first time at the International Philatelic Exhibition AMPHILEX 67 in Amsterdam. The first stamp of the Netherlands from 1852, one from 1864, and one from 1869 were reproduced on three new stamps. Because those first stamps were produced using plate printing, the three new stamps were manufactured using the same printing technique.
In 1976 and 1977, old stamps were reissued—first five, then four—all from long-running series featuring images of the various queens. But a foreign stamp was also reproduced on a Dutch stamp: the world-famous “Blue Mauritius,” which had been acquired by the then PTT Museum, now the Museum for Communication in The Hague.
The New Zealand Post also reproduced a first stamp from Mauritius. That was the orange one-penny stamp. On the occasion of the World Stamp Exhibition 'Philatelic Firsts', sheets were issued depicting many 'first stamps'.
But the most famous stamp is undoubtedly the British Penny Black, issued in 1840. This stamp was depicted by many countries on their own stamps. The British Post also did this a few times, the first time in 1970.
Want to start a collection alongside your main collection? Then consider a theme, such as 'stamps on stamps'. Hundreds of themes can be conceived. Is this something for you?

start a thematic collection?
then get ideas here:
http://thematische.verzamelgids.nl
I know an even nicer one: stamps on postcards. There are so very many of those...