Apparently, multiple colons are simply ignored when executing Ex commands. For example,
:set number! :::set number! : : :set number!
all toggle line numbers. Is this really a consolidated rule? Where is it documentated?
:helpgrep :: and
:help :: but did not find anything relevant.
I came to this question after many times having copied Ex commands such as
and pasted to Vim's command line with :CTRL+R*, so that
the resulting command line had a double colon.