Steven

I haven't seen your code but one thing you've got to watch out for is that the .ListCount changes as you add and remove.
This works for me, and in many applications (note the buttons are in the same container as the listboxes)

* cmdRemove.Click() - i.e. the single back-arrow button (<)

LOCAL	lnListCount

THISFORM.LockScreen 	    = .T.
nCnt = 1
With This.Parent
   DO WHILE nCnt <= .lstSelected.ListCount
	 IF .lstSelected.Selected( nCnt)
	 	lnListCount  = .lstSource.ListCount + 1
		.lstSource.AddListItem( .lstSelected.List( nCnt, 1), lnListCount, 1)
		.lstSource.AddListItem( .lstSelected.List( nCnt, 2), lnListCount, 2)
		.lstSelected.RemoveItem( nCnt)
	 ELSE
		nCnt 	    = nCnt + 1
	 ENDIF
  ENDDO
EndWith
THISFORM.LockScreen = .F.

* cmdRemoveAll (<<)

THISFORM.LockScreen 		= .T.
FOR nCnt = 1 to THIS.Parent.lstSelected.ListCount
	THIS.Parent.lstSource.AddListItem( THIS.Parent.lstSelected.List( nCnt, 1), nCnt, 1)
	THIS.Parent.lstSource.AddListItem( THIS.Parent.lstSelected.List( nCnt, 2), nCnt, 2)
ENDFOR
THIS.Parent.lstSelected.Clear
THISFORM.LockScreen 		= .F.

But the add buttons do it differently from yours:

* cmdAdd.Click() (>)
LOCAL	lnListIndex

THISFORM.LockScreen 		= .T.
* The ListCount value changes whenever the RemoveItem method is called
nCnt = 1
With This.Parent
  lnListIndex			= .lstSelected.ListCount
  DO WHILE nCnt <= .lstSource.ListCount
	  IF THIS.Parent.lstSource.Selected(nCnt)
		  lnListIndex 	= lnListIndex + 1
		  .lstSelected.AddListItem( .lstSource.List( nCnt, 1), lnListIndex, 1)
		  .lstSelected.AddListItem( .lstSource.List( nCnt, 2), lnListIndex, 2)
		  .lstSource.RemoveItem( nCnt)
	  ELSE
		  nCnt 		= nCnt + 1
	  ENDIF
  ENDDO
EndWith
THISFORM.LockScreen = .F.

* cmdAddAll.Click() (>>)
THISFORM.LockScreen = .T.
FOR nCnt = 1 to THIS.Parent.lstSource.ListCount
	THIS.Parent.lstSelected.AddListItem( THIS.Parent.lstSource.List( nCnt, 1), nCnt, 1)
	THIS.Parent.lstSelected.AddListItem( THIS.Parent.lstSource.List( nCnt, 2), +nCnt, 2)
ENDFOR
THIS.Parent.lstSource.Clear
THISFORM.LockScreen = .F.

HTH

Terry