# They must perform your duties

"They" refers to members of the tribe of Levi; the word "you" is singular and refers to Aaron. 

# perform your duties and the duties of the whole tent

"do the work I have commanded you to do and all the work connected with the whole tent." See how you translated similar words in Numbers 1:53.

# or they and also you will die

Here "they" refers to any member of the tribe of Levi who comes "near to anything in the sanctuary;" the word "you" is plural and refers to both Aaron and the rest of the Levites who are serving in approved roles.