There can be only 1 styles override for a combination of (module name) and (skin name).
If two extensions or skins try to define the same ResourceModuleSkinStyles override, then ExtensionProcessor should throw an exception.
Right now, when ExtensionProcessor is reading the second extension or skin, it is removing the styles from the first one.
This is a mistake. We should throw an error when this happens, so that a developer can take a look and decide which one should be removed.
This test could have helped prevent T167216.