Duplicate entries when loading several bib files

Question

I'm working on an edited volume, for the major time so far compiling the different chapters separately, each of them having (one or) two bibliographies. I have a biber.conf file for each chapter, for generating two bibliographies (primary and secondary literature (of which the primary ones are also an ugly hack, but that would be another question, so I've reduced my example here to one bibliography per chapter.)). I associate the bib-files with keywords, and then generate the bibliographies for primary and secondary literature based on that. The commands for that follow the pattern biber -g 1.conf 1.

I am now putting the volume together, using docmute, and am realising my biblatex setup is seriously broken. It would have been desirable to have just one bibliographical database, from which each of the contributions would feed and build up their respective bibliographies by \cite etc., but that's not how things have naturally grown here. I hope for the next volume I can do it like that. For now I guess I have to find a way to either keep the bibliographies completely separate, or make duplicate entries appear in all the bibliographies in whose bib files they are present.

In the following example (should be named 1.tex. a.tex and b.tex represent chapters of the volume, of which I only display the bibliographies here.) the entry A2015 is a duplicate. How can I make it appear in both bibliographies?

\documentclass{memoir}
\usepackage{filecontents}
\usepackage[authordate]{biblatex-chicago}
\usepackage{docmute}
\addbibresource{a.bib}
\addbibresource{b.bib}


\begin{filecontents*}{1.conf}
<?xml version="1.0" encoding="UTF-8"?>
<config>
  <sourcemap>
    <maps datatype="bibtex" bmap_overwrite="1">
      <map>
        <per_datasource>a.bib</per_datasource>
        <map_step map_field_set="KEYWORDS" map_field_value="a"/>
      </map>
      <map>
        <per_datasource>b.bib</per_datasource>
        <map_step map_field_set="KEYWORDS" map_field_value="b"/>
      </map>
    </maps>
  </sourcemap>
</config>
\end{filecontents*}

\begin{filecontents*}{a.tex}
\documentclass{memoir}
\usepackage[authordate]{biblatex-chicago}
\addbibresource{a.bib}
\begin{document}
\nocite{*}
\printbibliography[keyword=a,heading=subbibliography]
\end{document}
\end{filecontents*}


\begin{filecontents*}{a.bib}
@book{A2015,
author = {Some Body},
year = {2015},
title = {A book},
}
\end{filecontents*}


\begin{filecontents*}{a.conf}
<?xml version="1.0" encoding="UTF-8"?>
<config>
  <sourcemap>
    <maps datatype="bibtex" bmap_overwrite="1">
      <map>
        <per_datasource>a.bib</per_datasource>
        <map_step map_field_set="KEYWORDS" map_field_value="a"/>
      </map>
    </maps>
  </sourcemap>
</config>
\end{filecontents*}


\begin{filecontents*}{b.tex}
\documentclass{memoir}
\usepackage[authordate]{biblatex-chicago}
\addbibresource{b.bib}
\begin{document}
\nocite{*}
\printbibliography[keyword=b,heading=subbibliography]
\end{document}
\end{filecontents*}


\begin{filecontents*}{b.bib}
@book{A2015,
author = {Some Body},
year = {2015},
title = {A Book},
}

@book{B2015,
author = {Somebody Else},
year = {2015},
title = {Another Book},
}
\end{filecontents*}

\begin{filecontents*}{b.conf}
<?xml version="1.0" encoding="UTF-8"?>
<config>
  <sourcemap>
    <maps datatype="bibtex" bmap_overwrite="1">
      <map>
        <per_datasource>b.bib</per_datasource>
        <map_step map_field_set="KEYWORDS" map_field_value="b"/>
      </map>
    </maps>
  </sourcemap>
</config>
\end{filecontents*}

\begin{document}
\input{a}
\input{b}
\end{document}

Answer 1

If you want to keep the references in a.tex and b.tex completely separate (without a possibility for a proper common bibliography) you can use refsections.

You don't even have to touch the original .tex files it is enough to wrap the \input in a refsection environment where you have to specify the proper .bib file in the optional argument.

\begin{refsection}[a.bib]
\input{a}
\end{refsection}
\begin{refsection}[b.bib]
\input{b}
\end{refsection}

MWE

\documentclass{memoir}
\usepackage{filecontents}
\usepackage[authordate]{biblatex-chicago}
\usepackage{docmute}

\begin{filecontents*}{a.tex}
\documentclass{memoir}
\usepackage[authordate]{biblatex-chicago}
\addbibresource{a.bib}
\begin{document}
\nocite{*}
\printbibliography[heading=subbibliography]
\end{document}
\end{filecontents*}

\begin{filecontents*}{a.bib}
@book{A2015,
author = {Some Body},
year = {2015},
title = {A book},
}
\end{filecontents*}


\begin{filecontents*}{b.tex}
\documentclass{memoir}
\usepackage[authordate]{biblatex-chicago}
\addbibresource{b.bib}
\begin{document}
\nocite{*}
\printbibliography[heading=subbibliography]
\end{document}
\end{filecontents*}

\begin{filecontents*}{b.bib}
@book{A2015,
author = {Some Body},
year = {2015},
title = {A Book},
}

@book{B2015,
author = {Somebody Else},
year = {2015},
title = {Another Book},
}
\end{filecontents*}


\begin{document}
\begin{refsection}[a.bib]
\input{a}
\end{refsection}
\begin{refsection}[b.bib]
\input{b}
\end{refsection}
\end{document}

There are no Biber errors because the two refsections coexist independently.