Double line is displayed as a rectangle [duplicate]

Question

MWE:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node (x) {X};
\node[above right=1mm and 2mm of x] (y) {Y};

\draw[double,double distance=2mm] (x) -- (y);
\end{tikzpicture}
\end{document}

How to remove the thin lines?

Answer 1

Although I voted to close your question as duplicate (sorry), this is a different approach which give you a real, literal equal sign.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node (x) {$X$};
\node[above right=1mm and 2mm of x] (y) {$Y$};
\path (x) -- (y) node[midway,sloped] {$=$};
\end{tikzpicture}
\end{document}

Answer 2

Since this is a repeating question, here is a decoration that draws the double as two paths rather than a thick line with a white line on top, which is why you get the thin lines on the ends (for certain viewers, I think). The syntax is as simple as

\draw[alt double,alt double distance=2mm] (x) -- (y);

and also works for curved lines, see

\draw[alt double,alt double distance=2mm] (y) to[out=90,in=180] (z);

Here is the code.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{decorations,positioning} 
\pgfkeys{/tikz/.cd,
    alt double distance/.initial=5pt,
    alt double step/.initial=1pt,
}

\pgfdeclaredecoration{double deco}{initial}
{% initial arrow butt
\state{initial}[width=\pgfkeysvalueof{/tikz/alt double step},next state=cont] {
    \pgfmoveto{\pgfpoint{\pgfkeysvalueof{/tikz/alt double step}}{\pgfkeysvalueof{/tikz/alt double distance}/2}}
    \pgfpathlineto{\pgfpoint{0.3\pgflinewidth}{\pgfkeysvalueof{/tikz/alt double distance}/2}}
    \pgfpathmoveto{\pgfpoint{0.3\pgflinewidth}{-\pgfkeysvalueof{/tikz/alt double distance}/2}}
    \pgfpathlineto{\pgfpoint{1pt}{-\pgfkeysvalueof{/tikz/alt double distance}/2}}
    \pgfcoordinate{lastup}{\pgfpoint{1pt}{\pgfkeysvalueof{/tikz/alt double distance}/2}}
    \pgfcoordinate{lastdown}{\pgfpoint{1pt}{-\pgfkeysvalueof{/tikz/alt double distance}/2}}
  }
  \state{cont}[width=\pgfkeysvalueof{/tikz/alt double step}]{
     \pgfmoveto{\pgfpointanchor{lastup}{center}}
     \pgfpathlineto{\pgfpoint{\pgfkeysvalueof{/tikz/alt double step}}{\pgfkeysvalueof{/tikz/alt double distance}/2}}
     \pgfcoordinate{lastup}{\pgfpoint{\pgfkeysvalueof{/tikz/alt double step}}{\pgfkeysvalueof{/tikz/alt double distance}/2}}
     \pgfmoveto{\pgfpointanchor{lastdown}{center}}
     \pgfpathlineto{\pgfpoint{\pgfkeysvalueof{/tikz/alt double step}}{-\pgfkeysvalueof{/tikz/alt double distance}/2}}
     \pgfcoordinate{lastdown}{\pgfpoint{\pgfkeysvalueof{/tikz/alt double step}}{-\pgfkeysvalueof{/tikz/alt double distance}/2}}
  }
  \state{final}[width=0pt]
  { % perhaps unnecessary but doesn't hurt either
    \pgfmoveto{\pgfpointdecoratedpathlast}
  }
}
\tikzset{alt double/.style={decorate,decoration=double deco}}
\begin{document}
\begin{tikzpicture}
\node (x) {X};
\node[above right=1mm and 2mm of x] (y) {Y};
\draw[alt double,alt double distance=2mm] (x) -- (y);

\node[above right=1cm and 2cm of y] (z) {Z};
\draw[alt double,alt double distance=2mm] (y) to[out=90,in=180] (z);
\end{tikzpicture}
\end{document}

Using this decoration should also allow one to compute intersections with the double line.