[tex-eplain] Warnings about redefinition of xref labels

[Karl Berry]

    I would suggest not checking the value, but only
    the label to see if it was duplicated.

This seems sensible.  The whole goal is to warn about
\eqdef{foo}
...
\eqdef{foo}

And ditto for other xref defs.  And that's it.  Oleg ... ?

Thanks,
k