Tikz-decorations.markings标记位置问题

Question

为什么蓝线和红线与黑线不吻合，并且相互重叠？

\documentclass[tikz,margin=5pt]{standalone}

\usetikzlibrary{calc}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[Dble/.style args={#1 and #2}{%
decoration={%
    markings,% switch on markings
    mark=at position 0 with {%
        \node[yshift= .5\pgflinewidth] (Axx) {};
        \node[yshift=-.5\pgflinewidth] (Bxx) {};},
    mark=at position 1 with {%
        \node[yshift= .5\pgflinewidth] (Ayy) {};
        \node[yshift=-.5\pgflinewidth] (Byy) {};
        \draw [#1] (Axx)--(Ayy) ;
        \draw [#2] (Bxx)--(Byy) ;}
    },
    postaction={decorate}}
]

\coordinate (A) at (0,0) ;
\coordinate (B) at (1,1) ;

\draw[Dble={blue and red},very thick] (A)--(B) ;

\end{tikzpicture}
\end{document}

Answer 1

\documentclass[tikz,margin=5pt]{standalone}

\usetikzlibrary{calc}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[Dble/.style args={#1 and #2}{%
decoration={%
    markings,% switch on markings
    mark=at position 0 with {%
        \coordinate (Axx) at (0,.5\pgflinewidth);%
        \coordinate (Bxx) at (0,-.5\pgflinewidth);},%
    mark=at position 1 with {%
        \coordinate (Ayy) at (0,.5\pgflinewidth);%
        \coordinate (Byy) at (0,-.5\pgflinewidth);%
        \draw [#1] (Axx)--(Ayy) ;%
        \draw [#2] (Bxx)--(Byy) ;}%
    },%
    postaction={decorate}}
]

\coordinate (A) at (0,0) node {A};
\coordinate (B) at (1,1) ;

\path[Dble={blue and red},very thick] (A)--(B) ;

\end{tikzpicture}
\end{document}