sort -nu: bug or feature?

[Andrew Noymer]

sort -nu seems to compare only the first key, which is not what a lot of 
people expect.  I don't know if this is a bug, though.

THE DATA:

$ cat fish
1010    0
1010    1
1010    0

THIS works as we expect:

$ sort fish | uniq
1010    0
1010    1

SO does this:

$ sort -u fish
1010    0
1010    1

BUT I'm not really sure why making a numerical sort should bollox 
everything up:

$ sort -nu fish
1010    0

FWIW this does work:
$ sort -nu -k2 fish
1010    0
1010    1

BUT even this does not work:
$ sort -nu -k2,1 fish 
1010    0

Bug?  Feature? 

I see what's going on --- it's stopping comparison at the first key.

But I think most folks expect that "sort -nu" == "sort -n | uniq"
If this equality was never the intention, then "no problemo" as far as I 
am concerned.  But it seems to me that most folks assume the above 
equality is in effect.  On the one hand, a one-sentence clarification 
could be added to the man page?  On the other hand, if it's a bug, it 
could be fixed.

Thanks!

Andrew