rm feature: don't attempt to remove the same file twice

[Ed Avis]

% touch a
% rm a a
rm: cannot remove 'a': No such file or directory

It might be better for rm to remember that it has seen the filename 'a' before, 
and just warn rather than check the filesystem again.
This would be a bit more user-friendly and a bit faster, if you accept that 
filesystem operations are slower than doing user-space checking of seen 
filenames.
It would not catch 'rm a ./a' and similar cases but it would be an improvement.

-- 
Ed Avis <address@hidden>
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