“也” sounds like "yeah", can be translated as "too"
“又” sounds like "you", can be translated as "again"
you can say 他去过中国，我也去过中国 that's "he has been to china, and i have been there too".
他去年去过中国，今年又去了 that's"he has been to China last year,but he went there again this year."
or you can say 他去年去过中国，今年也去了 that's "he has been to China last year, and he went there again this year"
when you use "又" the emphasis is on the second time that what happen again, and it sound like the talker thinks that the same thing happen too much.
when you use "也" ,the talker might has no thought of "too much"