Quote - And, if you have two of the same prop, the Mat Pose usual ends up being applied to the first one, regardless of which catgirl collar you have selected.

That's probably because one of the props will be named something like "Collar_1", and as no two props in the scene can have the same name, the other collar prop will be named "Collar_2". If the mat file uses the name "Collar_1", then obviously it won't work on "Collar_2". A  way round this is to use "actor $CURRENT" in the MAT file in place of "prop Collar_1".