I changed the title in an attempt to make clear, that this designation is at least very speculative.
Perhaps an explanation for this designation can be found in volume 17 of the book series "Die deutsche
Luftfahrt - Willy_Messerschmitt-Pionier_der_Luftfahrt_und_des_Leichtbaues", page 193, as there are the
following lines in the paragraph about a proposed "construction kit aircraft" :
"Grundmuster III:
Schweres Kampfflugzeug, viermotoriges Hochleistungsflugzeug
(wie die Ju 290) mit Verwendungszweck als
- Fernaufklärer
- Fernkampfflugzeug"
That means, the basic type III should have been a heavy, 4-engined type like the Ju 290, to be used as long range
bomber, recce aircraft or fast long range transport. The project number for at least one of those construction kit aircraft
was P.1090, so the number 90 is mentioned at least twice and if read by someone with only cursory knowledge of that language,
an erroneous conclusion, that there was the "Me 290" designation seems not totally implausible to me.
And yes, there actually were cases, where designation numbers were used twice , as mentioned by Tophe, but such things were
very rare, I think. To my opinion, there's a much higher probability, that wrong interpretation of sources, maybe aggravated by
copying from one site to the next one, caused another "fake" designation. We had this here more than once and if "generated"
such things in most cases turn out to be very persistent !
So I would ask to be extra careful and base postings about this theme on reliable sources only !