this is Beijing accent. no need to know about this :D
倍:times 棒:good nice
倍棒: many times of 棒 it means very very very good
嘛:什么
吃嘛嘛香:吃什么 什么香 it means whatever you eat, you always feel that what you eat is very delicious.
anyway, all are Beijing accent.
however, this a form sentense: verb + 什么 什么+adj/adv
for example 吃什么什么香
干什么什么不行 whatever you do, you always fail....
the second 什么 represent the first 什么 and the first 什么 is a kind of vague word to represent anything.