Application of Brent's Cycle Detection Algorithm
If the numbers in the list are stored in memory as a list, you can just use a hashtable to look for duplicates, or you can sort them and then look for duplicates.

Brent's algorithm is primarily useful in situations where you don't want to store all of the numbers in memory, because the memory cost would be too high -- but that doesn't apply if you have a list with the numbers, because then you're already committed to storing all the numbers in memory.