Echo to both stdout and stderr

I have a probably pretty easy beginner question: How do I echo from a shell开发者_StackOverflow script into both stdout and stderr? I know that I can echo to stderr echo "foo" 1>&2 but I need the output in both. I tried some Googling but nothing worked.

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This should do it

 echo "foo" | tee /dev/stderr 

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echo foo | tee >(cat >&2)

This will work even if stderr is not supported.

Note: Soren's answer will not work if you have su'ed to another account that does have write permissions to the tty; but mine will.