And also this section in the part about Ghost Reviews:
So basically, if I were to get an item wrong and then right interchangeably, would I be able to reach level 100 with just one single review item? Or do I only get the XP for, say, getting from streak 7 to streak 8 once, even if I pass that multiple times for the same item?
You only receive xp the first time that you see a sentence for a grammar point and therefore only receive xp once per streak level. In other words, if you get a grammar point to streak 3 and receive 50 total xp, you will not be able to get that xp again. The only way that you can get that xp again is if you reset that item which would erase your xp for it anyway. Cheers!
Edit: I have updated the FAQ to hopefully make it a little more clear.