$++ 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.
Nowadays, $++ is useless, it theoretically has the same behavior as $+
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.