itch.io

amp_has_monster just checks whether that AMP slot isn't empty. There's no script to check whether the player has a specific monster, so you'd need to make a new script with content along these lines:

///amp_player_owns_monster(monID)
//Returns true if the player owns at least 1 monster of the given species, false otherwise.
for(var c = AMP_FIRST_ACTIVE; c < AMP_FIRST_ACTIVE + PARTYSIZE_ACTIVE + PARTYSIZE_BOXED; c++){
    if(global.active_monster_party[c,amp_MONID] == argument0){
        return true;
    }
}
return false

Using that script instead of amp_has_monster should give the result you want :)