[ruby-core:91434] [Ruby trunk Feature#15590] Add dups to Array to find duplicates

Issue #15590 has been updated by tad (Tadashi Saito).


How about `Set#add?` ?

```ruby
require 'set'

a = ["Tokyo", "Paris", "London", "Miami", "Paris", "Orlando", "Dubai", "Tokyo", "Paris"]
s = Set.new
p a.select{|e| !s.add?(e)} #=> ["Paris", "Tokyo", "Paris"]
```

----------------------------------------
Feature #15590: Add dups to Array to find duplicates
https://bugs.ruby-lang.org/issues/15590#change-76693

* Author: xdmx (Eric Bloom)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Many times I find myself debugging data and the need of finding duplicated values inside of an Array.

Based on the amount of data it could be a simple `array.detect { |value| array.count(value) > 1 }` or a more performant way like

```ruby
def dups_for(array)
  duplicated_values = []
  tmp = {}
  array.each do |value|
    duplicated_values << value if tmp[value]
    tmp[value] = true
  end
  duplicated_values
end
```
 
It would be awesome if there was a way directly from the core language to call `dups` (or another name, as it could be too similar to the current `dup`) on an array in order to get all the duplicated values.

I'd love to create a PR for this, but my C level is non-existent 




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