\documentclass{article}
%\usepackage[subsectionbib]{bibunits}
\usepackage{bibunits}
%\usepackage[globalcitecopy]{bibunits}
\begin{document}
\bibliographyunit[\section]
\defaultbibliographystyle{plain}
\section{First section}
some text \cite{Lamport:1994} more text more citations
\putbib[lit]

some text between the units
\defaultbibliographystyle{abbrv}
\section{Second and last section}
some text \cite*{Knuth:1991} more text more citations
\putbib[lit]

\bibliographystyle{alpha}
\bibliography{lit}
\end{document}
