Instances of Background Boxes keep increasing with each Chapter/function call

Question

I'm using the following MWE to create a PDF and when I edit the created PDF it reveals that with each chapter or \chapframe call the number of instances of boxes keep increasing. This can also be made visible by changing opacity and with the same color each chapter box gets darker.

Is there any way to solve this so that each graphical element is only placed once?

MWE is from here

\documentclass{book}
\usepackage[
  scale=1,
  angle=0,
  opacity=1,
  contents={}
]{background}
\usetikzlibrary{calc}
\usepackage{lipsum}

\pagestyle{plain}

% auxiliary counter
\newcounter{chapshift}
\addtocounter{chapshift}{-1}

% the list of colors to be used (add more if needed)
\newcommand\BoxColor{%
  \ifcase\thechapshift blue!30\or red!30\or olive!30\or magenta!30\else yellow!30\fi}

% the main command; the mandatory argument sets the color of the vertical box
\newcommand\ChapFrame{%
  \AddEverypageHook{%
    \ifodd\value{page}
      \backgroundsetup{contents={%
        \begin{tikzpicture}[overlay,remember picture]
          \node[
            fill=\BoxColor,
            inner sep=0pt,
            rectangle,
            text width=2cm,
            text height=4cm,
            align=center,
            anchor=north east
          ] 
          at ($ (current page.north east) + (-0cm,-2*\thechapshift cm) $) 
          {\rotatebox{90}{\hspace*{.3cm}\parbox[c][1.5cm][t]{3.4cm}{%
              \raggedright\textcolor{black}{\scshape\leftmark}}}};
        \end{tikzpicture}%
      }%
    }  
    \else
      \backgroundsetup{contents={%
        \begin{tikzpicture}[overlay,remember picture]
        \node[
          fill=\BoxColor,
          inner sep=0pt,
          rectangle,
          text width=2cm,
          text height=4cm,
          align=center,
          anchor=north west
        ] 
        at ($ (current page.north west) + (-0cm,-2*\thechapshift cm) $) 
        {\rotatebox{90}{\hspace*{.3cm}\parbox[c][1.5cm][t]{3.4cm}{%
            \raggedright\textcolor{black}{\scshape\leftmark}}}};
        \end{tikzpicture}%
      }%
    }  
    \fi
  \BgMaterial}%
  \stepcounter{chapshift}%
}

% redefinition of \chaptermark to contain only the title
\renewcommand\chaptermark[1]{\markboth{\thechapter.~#1}{}} 

\begin{document}

\chapter[intro]{Introduction}
\ChapFrame
\lipsum[1-7]

\chapter{Results}
\ChapFrame
\lipsum[1-7]

\chapter{Discussion}
\ChapFrame
\lipsum[1-7]

\end{document}