tableofcontents prints \leftmark and \rightmark

Question

I use fancyhdr for my headers and footers. This works well on all pages except for the Contents page. Here the header displays leftmark and rightmark, Contents is displayed twice. Any idea why this is the case?

\documentclass[12pt,twoside]{book}

% set margin

\usepackage[paper=a4paper,left=25mm,right=25mm,top=25mm,bottom=40mm,bindingoffset=6mm]{geometry}

\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{booktabs}
\usepackage{totcount}
\usepackage{lastpage}
\usepackage{float}

% headers and footers (https://www.youtube.com/watch?v=mZcV1wIPCBo)

\usepackage{fancyhdr}

% beautiful cross-references with hyperref

\usepackage{xcolor,colortbl}
\definecolor{Green}{RGB}{121,141,33}
\usepackage{hyperref} % http://ctan.org/pkg/hyperref

\hypersetup{
  colorlinks,
  linkcolor=Green,
  citecolor=Green,
  filecolor=Green,
  urlcolor=Green,
}

% change font

\usepackage[default,osfigures,scale=0.95]{opensans}
\usepackage[T1]{fontenc}
% \usepackage[sfdefault,light,condensed]{roboto}  %% Option 'sfdefault' only if the base font of the document is to be sans serif
% \usepackage[sfdefault]{AlegreyaSans}
% \renewcommand*\oldstylenums[1]{{\AlegreyaSansOsF #1}}

%disable indent globally

\setlength{\parindent}{0pt}

\begin{document}
  \input{"0"}

  \pagestyle{fancyplain}
  \fancyhf{}
  \fancyhead{}
  \fancyfoot{}
  \fancyhead[LO,RE]{\rightmark}

  \tableofcontents

  \fancyhead{}
  \fancyfoot{}
  \fancyhead[LE,RO]{\leftmark}
  \fancyhead[LO,RE]{\rightmark}
  \fancyfoot[LE,RO]{my footer\\~\\Page~\thepage~of~\getpagerefnumber{LastPage}}
  \fancyfoot[LO,RE]{my footer}
  \renewcommand{\headrulewidth}{0.4pt}
  \renewcommand{\footrulewidth}{0.4pt}
  \makeatletter
  \let\ps@plain\ps@fancy
  \makeatother

  \input{"Chapters/1"}
  \input{"Chapters/2"}
  \input{"Chapters/3"}
  \input{"Chapters/4"}
  \input{"Chapters/5"}
  \input{"Chapters/6"}
\end{document}

Answer 1

It's a problem of timing, due to the changes of page style you're doing. Issuing

\tableofcontents
\cleardoublepage

will fix the problem.

Your main problem is, however, confusing the various planes and of giving contradictory instructions. It's better sticking with simple page styles and not defining them inside the document.

Here's a full version with changes. Note the option heightrounded to geometry, with also a setting for headheight as requested by fancyhdr.

I define two page styles: one for the front matter, one for the main matter, which are automatically selected by the corresponding commands. The initial part (your \input{0}) uses the empty page style. The \chapter command is modified so it doesn't issue the \thispagestyle{plain} instruction.

\documentclass[12pt,twoside]{book}

% set margin
\usepackage[
  paper=a4paper,
  left=25mm,
  right=25mm,
  top=25mm,
  bottom=40mm,
  heightrounded,
  headheight=14.5pt,
  bindingoffset=6mm,
]{geometry}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{booktabs}
\usepackage{totcount}
\usepackage{lastpage}
\usepackage{float}
\usepackage{etoolbox} % for \patchcmd
% headers and footers (https://www.youtube.com/watch?v=mZcV1wIPCBo)
\usepackage{fancyhdr}

\usepackage[default,osfigures,scale=0.95]{opensans}

\usepackage{lipsum} % just for this example

% beautiful cross-references with hyperref
\usepackage{xcolor,colortbl}

\usepackage{hyperref} % http://ctan.org/pkg/hyperref

\hypersetup{
  colorlinks,
  linkcolor=Green,
  citecolor=Green,
  filecolor=Green,
  urlcolor=Green,
}

%%% customizations
\definecolor{Green}{RGB}{121,141,33}

\pagestyle{fancy}

\fancypagestyle{frontmatter}{%
  \fancyhf{}%
  \fancyhead[RO,LE]{\rightmark}%
}
\fancypagestyle{body}{%
  \fancyhf{}%
  \fancyhead[LE,RO]{\leftmark}%
  \fancyhead[LO,RE]{\rightmark}%
  \fancyfoot[LE,RO]{my footer\\~\\Page~\thepage~of~\getpagerefnumber{LastPage}}%
  \fancyfoot[LO,RE]{my footer}%
  \renewcommand{\headrulewidth}{0.4pt}%
  \renewcommand{\footrulewidth}{0.4pt}%
}

\patchcmd{\chapter}{\thispagestyle{plain}}{}{}{} % remove \thispagestyle{plain}
%%% continuous numbering
\makeatletter
\renewcommand\frontmatter{%
  \cleardoublepage
  \pagestyle{frontmatter}%
  \@mainmatterfalse
}
\renewcommand\mainmatter{%
  \cleardoublepage
  \pagestyle{body}%
  \@mainmattertrue
}
\makeatother

\begin{document}

\pagestyle{empty}% titlepage
%  \input{"0"}
\lipsum[1]

\frontmatter
\tableofcontents

\mainmatter

\chapter{Title}
\lipsum
\chapter{Title}
\lipsum
\chapter{Title}
\lipsum
\chapter{Title}
\lipsum

%  \input{"Chapters/1"}
%  \input{"Chapters/2"}
%  \input{"Chapters/3"}
%  \input{"Chapters/4"}
%  \input{"Chapters/5"}
%  \input{"Chapters/6"}
\end{document}