Add Duplicate & DuplicateBy

[CorentinClabaut]

It would be useful to have:
func Duplicate[T comparable](collection []T) []T
func DuplicateBy[T any, U comparable](collection []T, iteratee func(T) U) []T

That would only return the duplicated values of the collection
.

[CorentinClabaut]

@samber what do you think about that?

I know it's not usually a standard function but I thought it could be useful.
Stackoverflow contains a lot of questions about finding duplicates.

In my case it was useful to simply validate some input and write an error with the duplications if any were found.