AdiIRC

This is incorrect, -a appends, if you want to replace, use -ra, -a should not overwrite.

Try it

dialog testbox {
  title "iii" 
  size -1 -1 100 100
  option dbu
  box "text", 1, 5 5 90 90
}

alias testbox {
  dialog -m testbox testbox
  did -a testbox 1 text2
  echo -ag $did(testbox, 1).text
}

Right, mIRC is wrong then.