I tried aggregate, apply etc, but can't get the right result. 
 
For example, 
 
m <- cbind(c(LETTERS[1:5]), c("aa", "bb", "cc", "aa", "cc"))     [,1] [,2][1,] "A"  "aa"[2,] "B"  "bb"[3,] "C"  "cc"[4,] "D"  "aa"[5,] "E"  "cc"
how to obtain m.new where "aa", "bb", and "cc" are groups, and more than one values "A" to "E" belonging to the same group are separated by "|"
       [,1] [,2][1,] "aa"  "A|D"[2,] "bb"  "B"[3,] "cc"  "C|E"
 
 
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