Repeated DNA Sequences

Statement

Given a string, dna, that represents a DNA subsequence, and a number $k$, return all the contiguous subsequences (substrings) of length $k$ that occur more than once in the string. The order of the returned subsequences does not matter. If no repeated substring is found, the function should return an empty set.

The DNA sequence is composed of a series of nucleotides abbreviated as $A$ ...