$++ is a very old construct, similar to $+ concatenator, which allowed a 'delay' to the $+ behavior. The example :
" $+ $read c:\test.txt $++ "
can be found in the changelog of mIRC, suggesting that if that $+ were used instead of a $++, it would concatenate the " to the filename before the file could be read but nowadays the above example works with $+ and $++
However $++ does differ from $+, inside brackets. As you may know, $+ inside bracket evaluation [ ] is not really behaving like it is outside them, $++ will act as though $+ was used, but it will behave the way $+ does outside the bracket evaluation, for example [ $ $++ me ] will produce plain text $me instead of [ $ $+ me ] producing your nickname
Added: mIRC v4.1
Added on: 27 Apr 1996
Note: Unless otherwise stated, this was the date of original functionality.
Further enhancements may have been made in later versions.