In normalize.css, the monospace font rules contain
font-family: monospace, monospace;
Is this different to
font-family: monospace;
?
There must be a reason for using that. Maybe it's a workaround for the behaviour of some browsers?
You are right. The font-family: monospace, monospace; declaration is a simple hack/workaround for some browsers which tend to reduce the font size of monospace fonts.
More info on this Github issue: https://github.com/necolas/normalize.css/issues/519#issuecomment-197131966