I'm trying to add the > and >> on the two sets of parallel lines and having no luck.
\documentclass{scrreprt}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=1,xscale=2]
\coordinate (A) at (0,0);
\coordinate (B) at (2,2.5);
\coordinate (C) at (4,2.5);
\coordinate (D) at (2,0);
\draw[<->,thick,shorten >=-60pt,shorten <=-60pt] (A) -- (B)node[midway]{SINGLE};
\draw[<->,thick,shorten >=-60pt,shorten <=-60pt] (C) -- (D)node[midway]{SINGLE};
\draw[<->,thick,shorten >=-60pt,shorten <=-60pt] (A) -- (D)node[midway]{DOUBLE};
\draw[<->,thick,shorten >=-60pt,shorten <=-60pt] (B) -- (C)node[midway]{DOUBLE};
\end{tikzpicture}
\end{document}