In the current development snapshot (as of 2013-07-22) compiled on Ubuntu 12.04 the epslatex terminal ouputs lines like
\newlength{\gptboxheight}% \newlength{\gptboxwidth}%
which might be correct for single plots but leads to errors ("LaTeX Error: Command \gptboxheight already defined.") when including several plots in one document (e.g. with the LaTeX gnuplottex package).
Good point. Please try today's CVS version. I have tried to wrap the offending definitions in a conditional test for a previous definition. But I'm not a TeX guru so I cannot promise that this solution works for all LaTeX flavors.
If this fix does not work, another option is to move the definition into the file gnuplot.cfg.
That's working! Thanks for your quick response.