\makeatletter
% We'll keep track of the old/seen bibkeys in this list.
\def\@tufte@old@bibkeys{}

% prevent url, doi, isbn, issn from appearing in margin-citations.
\AtEveryCite{%
 \DeclareFieldFormat{url}{}%
 \DeclareFieldFormat{urldate}{}%
 \DeclareFieldFormat{doi}{}%
 \DeclareFieldFormat{isbn}{}%
 \DeclareFieldFormat{issn}{}%
}%


% This macro prints the full citation if it's the first time it's been used
% and a shorter citation if it's been used before.
\newcommand{\@tufte@print@margin@citation}[1]{%
  % print full citation if bibkey is not in the old bibkeys list
  \xifinlist{#1}{\@tufte@old@bibkeys}{%
    \cite{#1}% print short entry
  }{%
    \fullcite{#1}\unskip% print full entry
    % add bibkey to the old bibkeys list
    \listxadd{\@tufte@old@bibkeys}{#1}%
  }%
}%

% We've modified this Tufte-LaTeX macro to call \@tufte@print@margin@citation
% instead of \bibentry.
\renewcommand{\@tufte@normal@cite}[2][0pt]{%
  % Empty variable for last bibkey, 
  \let\@temp@last@bibkey\@empty%
  % Snag the last bibentry in the list for later comparison
  \@for\@temp@bibkey:=#2\do{\let\@temp@last@bibkey\@temp@bibkey}%
  \sidenote[][#1]{%
    % Loop through all the bibentries, separating them with semicolons and spaces
    \normalsize\normalfont\@tufte@citation@font%
    \setcounter{@tufte@num@bibkeys}{0}%
    \@for\@temp@bibkeyx:=#2\do{%
      % Check if we're already dealing with the last key in the list of to-cite keys.
      \ifthenelse{\equal{\@temp@last@bibkey}{\@temp@bibkeyx}}{%
        % Add "and " to the last one. E.g. a; b; c; and d.
        % unless there was only 1 (checked with num@bibkeys).
        \ifthenelse{\equal{\value{@tufte@num@bibkeys}}{0}}{}{and\ }%
        \@tufte@trim@spaces\@temp@bibkeyx% trim spaces around bibkey
        \@tufte@print@margin@citation{\@temp@bibkeyx}%
      }{%
        \@tufte@trim@spaces\@temp@bibkeyx% trim spaces around bibkey
        \@tufte@print@margin@citation{\@temp@bibkeyx};\space%
      }%
      \stepcounter{@tufte@num@bibkeys}%
    }%
  }%
}%

\newcommand{\@tufte@normal@cite@separate}[2][0pt]{%
  \@for\@temp@bibkeyx:=#2\do{%
    \sidenote[][#1]{%
      % Loop through all the bibentries, separating them with semicolons and spaces
      \normalsize\normalfont\@tufte@citation@font%
      \@tufte@trim@spaces\@temp@bibkeyx% trim spaces around bibkey
      \@tufte@print@margin@citation{\@temp@bibkeyx}%
    }%
  }%
}%

%\renewcommand{\cite}[2][0pt]{\sidenote[][#1]{\fullcite{#2}}}% for biber / biblatex
\newcommand{\paulciteseparate}[2][0pt]{\@tufte@normal@cite@separate[#1]{#2}}% 
%\newcommand{\paulcite}[2][0pt]{\sidenote[][#1]{\fullcite{#2}}}% 
\newcommand{\paulcite}[2][0pt]{\@tufte@normal@cite[#1]{#2}}% 

% Calling this macro will reset the list of remembered citations. This is
% useful if you want to revert to full citations at the beginning of each
% chapter.
\newcommand{\resetcitations}{%
  \gdef\@tufte@old@bibkeys{}%
}
\makeatother

%%% Local Variables:
%%% mode: latex
%%% TeX-master: "paul-thesis"
%%% End: