xrefwarn – Only one warning for an undefined citation
The package patches the citation mechanism so that you get at most one warning about each reference. (Without the package, you get one warning per citation, whether or not the reference has already been cited.)
Since the package works on internal macros, other citation packages may not treat it kindly.
Maybe you are interested in the following packages as well.
- cite: Improved citation handling in LaTeX
- makerobust209: Make an existing LaTeX command robust
- mflogo209: Typeset the METAFONT logo under LaTeX 2.09
- sfwmac: A set of definitions for Unix system documentation