Thanks for your post Klatuu

Where should I put this SQL?  You mention it should go in the After Update
event of the control I am using to select a warning.  I pasted it into the
after update event procedure and the SQL was coloured red (this must be the
wrong place to put it... this is just for VB, right?)  
I am using the combo to select warnings (in the detail section of a
continuous subform) and so I want only warnings that aren't already assigned
to appear in the combo (so does this query need to go in the rowsource for
the combo?)  Presumably I would then put the requery in the after update
event of the combo?

The message box idea is fine if it works.  But I don't see why it would be
needed.  If the combo on each record in the subform only included warnings
that weren't already selected, it would be impossible to select a duplicate.  
That is the thinking behind all of this.  If there is another way of doing
this, fine.

Do I need to create a query outside of the combo box, give it a name and
refer to this query name as the row source for the combo, or can I put it
directly into the combo itself?  (Does this make any difference?  What's
confusing me is the fact that the combo has to "requery" after each
selection.)

Throughout all of this I have been unclear on where to put the SQL.  Surely
excluding the selected warnings from rowsource for the combo is the key to
sorting this....?   What am I doing wrong?  The textbox idea was to put a
textbox carefully positioned directly over the combo so that it would still
display the previously selected warning (using DLookup) for that driver
(which would now not show up as it would be excluded from the row source.)

Have I explained all this OK?
Looking forward to your thoughts on this one
Rich