natbib: Error when citing paper with similar authors and same year

The error occurs because of the double braces in the bib file (especially in cases, where just a number is expected). Why do you have double braces around nearly every field in the bib file? If I change them to single braces, everything works fine.

\documentclass[a4paper]{article}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{mieghem10assortativity,
        Month = {Nov},
        Numpages = {11},
        Publisher = {American Physical Society},
        author = {Van Mieghem, P. and Ge, X. and Schumm, P. and Trajanovski, S. and Wang, H.},
        title = {Spectral graph analysis of modularity and assortativity},
        journal = {Physical Review E},
        volume = {82},
        number = {5},
        pages = {056-113},
        year = {2010},
        doi = {10.1103/PhysRevE.82.056113},
}

@article{mieghem10rewiring,
        author = {Van Mieghem, P. and Wang, H. and Ge, X. and Tang, S. and Kuipers, F. A.},
        title = {Influence of assortativity and degree-preserving rewiring on the spectra of networks},
        journal = {European Physical Journal B},
        volume = {76},
        number = {4},
        pages = {643-652},
        year = {2010},
        doi = {10.1140/epjb/e2010-00219-x},
}
\end{filecontents}
\usepackage[numbers]{natbib}

\begin{document}
blah. \citep{mieghem10assortativity,mieghem10rewiring} says
blah blah

\bibliographystyle{plainnat}
\bibliography{\jobname}

\end{document}

(Note: the filecontents package and environment are just for producing the bib file, they are not part of the solution.)