I have the following array
[1, 2, 3, 4, 5, 1, 2, 5, 3, 4, 2, 3, 1, 3, 2, 2]`
array
[2, 3, 1]
arr = [1, 2, 3, 4, 5, 1, 2, 5, 3, 4, 2, 3, 1, 3, 2, 2]
= arr.group_by { |e| e }.map { |e| e[0] if e[1][1] }.compact
[1, 2, 3, 4, 5]
To get a count of duplicated entries per duplicate you can go with:
arr.group_by(&:itself)
.each_with_object({}) {|(k, v), hash| hash[k] = v.size }
#=> {1=>3, 2=>5, 3=>4, 4=>2, 5=>2}
To get 3 most duplicated entries:
arr.group_by(&:itself)
.sort_by { |_k, v| -v.size }
.take(3)
.map(&:first)
#=> [2, 3, 1]