latexmk and repeated bibtex entries

[taiya]

I've noticed the current latexmk gives a truly cryptic warning when bibtex entries are repeated?
Compiling with CMD+R results in:

Command latexmk -pdf -f -r /tmp/latexmkrc -r '/Users/andrea/Library/Application Support/TextMate/Managed/Bundles/LaTeX.tmbundle/Support/config/latexmkrc' espresso.tex exited with status 12

While using watchdocument (using latexmk), results in a completely uninformative popup
(and no trace of duplicated bibtex is present in the latekmk.log)