What is the meaning of "比一比" in 一件件新衣向身上比一比?
比 in most context means "to compare" e.g. 比大小 (compare the size) 

But in the sentence, "一件件新衣向身上比一比" , '比' means 'to cling to or 'lie next to' (in order to compare or match)'

It is very common for a person to pick up a new clothes and  'cling' it to his body to check if the size and the style is for him

You can see it more clearly without reduplication of '比'

"把那件新衣往身上比一下, 看看大小是否适合"