it takes at least two sessions to plant a word, or thing(depending on course). The sequence of the sessions is different on the app and the website.
On the website you plant 5 words (let us call this set "A") during one session. The next session gives you other words "B". If you continue you will get all the words from "A" again (this might be the "problem" you described). Then you get new words "C", then you repeat "B", then new "D", then repeat "C", new "E",... and so so on. In case you got a word wrong this will be asked one more time which leads to the sets being mixed during a session.
On the app I experienced a simpler sequence: Plant new set "A", repeat "A", new set "B", repeat "B",...
But all the words are asked multiple times and at least in two different sessions.