Bad ligatures with fontenc

[Gavin Smith]

In the LaTeX output, the fontenc package is used as

\usepackage[T1]{fontenc}

I found there was bad formatting for a string of commas in a fixed width
font.  This is a minimal example:

\documentclass{book}
\usepackage[T1]{fontenc}

\begin{document}

\texttt{abc,,,,xyz}
\end{document}

I've attached the output as an image "clip.png".  If I remove the fontenc
line, it looks like "clip2.png".

It appears in whatever font fontenc is using, there is a ligature that
transforms ,, into a single character.

clip.png
Description: PNG image

clip2.png
Description: PNG image