[lkrubner]

Neutral mutations can simply be ignored for answering the question posed above. Gort asked:

"isn't it possible, at least in theory, that mutation happened more than once?"

The answer is obviously "yes". Even taking your own words, the answer is clearly "yes", so I'm not clear why you are arguing. Even in the extreme case, where mutations lead to death 99.9999% of the time, the answer would remain "yes". Your own math shows the answer is "yes". I'm sure you know what convergent evolution is:

http://www.zo.utexas.edu/courses/thoc/convergence.html

As it says there:

"Molecules can evolve convergently"

I think you wanted to make the point that this is rare. You should have said so. You could have answered gort by saying "Yes, but this is rare." Instead, you've taken an indefensible position.