List of words in Possession is wrong
In the "Possession" group, there are words listed in the descriptions like "twojemu", "waszym", "mojej", "waszego", etc... but they never appear in the actual lesson. At least not for me.
This is because the words are added as entire sets of forms, which means they technically also include forms which are taught later. In practice, some forms do not appear immediately, even though they are classified as belonging to an earlier skill – they are introduced in skills that feature the respective case. So “twojemu” (Dative form of “twój”) will appear in the skill where the Dative case is introduced.
So you shouldn't worry, it is done this way to optimize the learning process. We would rather not see the more “advanced” forms displayed in early skills, but this is how Duo works, there is nothing we can do about it.
Ok thanks for your answer, I'm not really used to the way Duolingo works!
Do you mean in the list of words in individual lessons? It's my understanding that you're unlikely to hit every word/sentence within any given lesson on your first run through it. I know I'm still seeing new sentences even in languages I've been learning for ages.
(I've definitely seen mojej in sentences, and I think I've seen waszego... not having bumped into them =/= them not existing in the lesson ;))