find the repeated elements in a list
FindRepetitions(L, N, f, opt1, opt2, ...)
(optional) non-negative integer or infinity
opt1, opt2, ...
(optional) extra arguments to f
The FindRepetitions(L) function inspects the elements in list L and returns all the elements which previously appeared in the list.
If the optional second argument is specified, FindRepetitions(L, N) removes N copies of each unique element in list L (if possible).
By default, operands are compared by using boolean comparison. If a third argument is specified, then the function f(x, y, opt1, opt2, ...) is called to check if x and y should be considered equal. This function should implement an equivalence relation (i.e. it should be reflexive, commutative, and transitive). If it is not, then the result may not be valid.
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