Puzzle of repeating numbers

My friend Siva asked me this puzzle.
There is a set of N numbers. All numbers in this set repeat even number of times, except two numbers that repeat odd number of times. Develop an algorithm that will find these two numbers in O(N) space & time complexity.

I couldn't find the solution, but the solution Siva gave and another variant of that solution that his friend gave were simply amazing. Not to spoil the fun, I am not giving any of those solutions here :-)

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