Code:
on *:START:{ hmake -s 50 longline | set %longlinecounter 1 }
on *:TEXT:*:[color:purple]#chanA,#chanB[/color]:{
  if ($len($1-) >= 300) {
    if ($hfind(longline,$1-,1).data) {
[color:orange]      if (%longlinetimeout > 4) {
        inc -z %longlinetimeout 3[/color]
        notice $nick Please don't repeat long lines
[color:orange]      } [/color]
    }
    else {
      inc %longlinecounter
      [color:blue]if (%longlinecounter > 1000000) set %longlinecounter 1[/color]
      hadd -u10 longline %longlinecounter $1-
    }
  }
}


Change the purple text to your own channel(s) you want to monitor.
Remove or replace the orange text with your own flood protection stuff if needed.
The blue text is only really needed if you have a very busy channel and mIRC runs 24/7 for weeks...
Also note that i have not put in a check if you have ops, it will work if you're not an op, but make sure you have permission from them before using this script in their channel...