Of course, every couple of days the bible has to totally repeat itself for several chapters.
Numbers 18 goes over what the priests and the Levites should do in relation to the tabernacle. If there are subtle differences in the instructions I can't find them. The second half of Numbers 18 is the offerings for priests and Levites, again. I think if I hear about offering the first born animals to God one more time I'm going to die (don't hold me to that).
Numbers 19 is about how to make the water of "cleansing". Here's how you make it, let me know how clean you think this water is. You slaughter a red heifer and you burn it whole (the bible specifically mentions you leave the intestines in). Then you throw some random wood, plants, and wool on the burning heifer. You then put the ashes in water and use it to wash yourself. Then you're clean? Metaphorically clean I guess.
At the beginning of Numbers 20 Moses creates water from a rock again. Not very creative Moses.
Israel tries to pass through Edom. They say that they will go through the main road and not touch anything (keep in mind there are 600,000 Israelis). Edom says no. Good call, God would probably have found some reason to kill them. You know, on that thought, I wonder why God doesn't immediately kill the people of Edom for not letting Israel pass. I guess God is staying out of this one.
At the end of Numbers 20 God kills Aaron. Maybe you could argue that he dies naturally but I don't think so. God orders Aaron to the top of the mountain saying "[Aaron] will die here". God then tells Aaron to take off his clothes and he dies. This is because Aaron rebelled against God, in relation to the water coming out of the rock. How? I re-read the beginning of Numbers 20 and I can't see how Aaron rebelled against God in any way.