You could add a +
specifier for the “repeat chapters”; the proposed code doesn't interfere with usages of \chapter*
.
\documentclass{book}
\makeatletter
\newcounter{repchapter}
\let\latex@chapter\chapter
\renewcommand{\chapter}{\@ifstar{\latex@chapter*}{\guuk@chapter}}
\newcommand{\guuk@chapter}{\@ifnextchar+{\repeat@chapter}{\restore@chapter\latex@chapter}}
\newcommand{\repeat@chapter}[1]{% #1 is +
\addtocounter{chapter}{-1}%
\addtocounter{repchapter}{1}%
\gdef\thechapter{\arabic{chapter}\alph{repchapter}}%
\latex@chapter
}
\newcommand{\restore@chapter}{%
\setcounter{repchapter}{1}%
\gdef\thechapter{\arabic{chapter}}%
}
\makeatother
\begin{document}
\tableofcontents
\chapter{Title}
x
\chapter+{Repeat}
x
\chapter{Title}
x
\chapter+{Repeat}
x
\chapter+{Again}
x
\end{document}

If you load hyperref
, then add
\renewcommand{\theHchapter}{\thechapter}
just after \makeatletter
. Make sure \usepackage{hyperref}
is before the shown code.
\renewcommand{\thechapter}{<whatever>}
before\chapter{<title>}
...