Melira's first ability creates a replacement effect. If another replacement effect would also change the number of poison counters a player receives, such as that of Vorinclex, Monstrous Raider, the player receiving the counters chooses which one to apply first, regardless of who controls what permanents. This will usually result in that player choosing to apply Melira's effect last and receive one counter. However, if they want to, they could apply Melira's replacement effect first and receive more counters.

Once Melira's replacement effect has been applied in a turn, additional events that would put poison counters on a player don't happen, regardless of any other replacement effects involved.