Duplicate Mods

A commonly overlooked but potentially game breaking issue with modding RimWorld are duplicate mods in the mods-folder. While each mod is stored directly in its own sub-folder, the game itself doesn't actually differentiate mods that way. That includes the fact that the name of the mod-folder doesn't matter at all as far as the game is concerned. What actually defines the identity of any single mod is its packageID which is defined by the author in the mod's About.xml-file.

Because of that, mods sharing the same packageID (often the case with different versions/forks of the same mod, older versions and continuations, etc.) are seen by the game as the SAME mod. When the game starts it reads over the ModsConfig.xml to define which mods it has to load. That file is basically just a list of the packageIDs of the mods you have currently enabled so if you have two mods sharing the same packageID in your mods-folder, the game will (roughly speaking) pick the first mod-folder it associates that packageID with, which can at times even be the copy of the mod you technically didn't enable.

Obviously that means that you always should unsubscribe from any mods you have in a different version - no matter what. Otherwise there's always a risk the game will outright load mod-files from the wrong version and there's no clear communication of the game telling you which version it actually loaded. So at best you will realize that if you encounter errors or issues that only make sense to happen with the incorrect version of that mod. The game will inform you about those duplicates in general though, directly in the log. It will do so even if BOTH copies of the mods are currently inactive on your modlist.

But even if you don't use either copy right now, the best practice is to unsub/delete the version of the mod you surely don't need (usually this means the older version) because at some point in time you might re-enable one of them again - not realizing you have a dupe sitting around - and then run into issues which can at times be save-breaking even.

Keep your setup clean!

One additional problem with duplicate mods is widely unknown by people as well. If you're using two mods with the same packageID - one from the Workshop and the other one in your local Mods-folder - the game itself appends the packageID of the Steam version to differentiate between them (mod.package.id_steam.) Since mods use the packageID to optionally load content for cross-compatibility, that also means that a mod looking for a certain other mod by (mod.package.id) wouldn't see that mod if it has been modified that way. Thus the compatibility-patches wouldn't be loaded at all.

There is some action being taken by a few mod-authors to cover for that possibility and some mods are 'fixing' this behaviour globally to avoid those cases, but best practice clearly is to never have duplicate mods in your different mod-folders to begin with!

WIP!