You may test a boolean at ActivateForm:
I use Code like this myself:

Var
UpdList:Boolean;

On Create:
IBEvent.Registered := True;
UpdList := True;

ON Event:
If not Form.Visible Then UpdList := True
else
 Tabsheet.Refresh;

On FormActivate:
If (Form.Visible and UpdList) Then Tabsheet.Refresh.
UpdList := False;

On Destroy:
IBEvent.Registered := False;
Hope you can use this small exampel.

Regards Thomas Olsen.