On one of my Pokemon games I've used the PS to obtain the event Mew... Now I'm wondering if I can still redeem the code of that pokemon in the same game?
Nope.
The alternative is, if you have backed up a save before you collected the code,
you can revert to that save.
However, do know that the server logs the entry.
So even if you had another code, you won't be able to collect Mew again with the same cartridge.
The way to redeem another pokémon with the same code is to backup/restore your save right before you redeem the pokémon to the NPC.
Both pokémon won't have the same nature/IV, so you can consider them as no clone , and no hack if you get them via official way.
\o/