Try this:
val dup = List(1,1,1,2,3,4,5,5,6,100,101,101,102)
dup.groupBy(identity).collect { case (x, List(_,_,_*)) => x }
The groupBy associates each distinct integer with a list of its occurrences. The collect is basically map where non-matching elements are ignored. The match pattern following case will match integers x that are associated with a list that fits the pattern List(_,_,_*), a list with at least two elements, each represented by an underscore since we don’t actually need to store those values (and those two elements can be followed by zero or more elements: _*).
You could also do:
dup.groupBy(identity).collect { case (x,ys) if ys.lengthCompare(1) > 0 => x }
It’s much faster than the approach you provided since it doesn’t have to repeatedly pass over the data.