[PKMN RB] Make Encounters in one location unique

[palex00]

Please format your title with what portion of the project this pull request is targeting and what it's changing.

ex. "MyGame4: implement new game" or "Docs: add new guide for customizing MyGame3"

What is this fixing or adding?

Previously in generation one Pokémon could occupy multiple encounter slots in one location. The player was thus unable to determine whether they encountered / caught all Pokémon on the route. This changes it so there will be unique encounters.

Code is by @vyneras who allowed me to PR it

How was this tested?

I did ten test generations with 1:1 mapping set to false and various settings regarding pokemon types, base stats and randomizations and checked both with a script and manually whether there are duplicates. There were not.

If this makes graphical changes, please attach screenshots.

N/A