Is code duplication allowed in this instance?

[Klone]

Hi all. I believe in programming there is a common consensus to avoid
code duplication, I suppose such terms like 'DRY' are meant to back
this idea. Anyways, I'm working on a little project and I'm using TDD
(still trying to get a hang of the process) and am trying to test the
functionality within a method. Whoever it so happens to verify the
output from the method I have to employ the same algorithm within the
method to do the verification since there is no way I can determine
the output before hand.

So in this scenario is it OK to duplicate the algorithm to be tested
within the test codes or refactor the method such that it can be used
within test codes to verify itself(??).