Some months ago I planted the seeds from a papaya fruit I bought from supermarket (UK).
I used new soil and I got about 30 seedlings, all looking the same (so it is very unlikely they are some unknown spare seeds came from nowhere, also some of them had the seed capsule still on).
I selected 4 seedling and planted in a large pot.
This is the result, and they don't look like papaya plans at all. Can someone tell me what they are and how it is possible they were inside a papaya fruit?

No idea - what did the seeds look like? Could you send us some photos when it flowers?

No those aren't Papayas. Did you maybe grow a lot of pepper seeds too?

Papaya seeds are round and black, like pepper corms. Pepper seeds are flat and white.

All polycot seedlings look alike: two little cotyledons (seed leaves). But Papaya cotyledons are round while pepper cotyledons are pointed.