I have solved It. I created a compound unique key of both FK and PK of JoinTable as suggested, and added this code in Before_update event
Code:
Dim Duplicate As Long
If Me.Dirty = True Then
If Not IsNull(Combo5) Then
Duplicate = Nz(DLookup("ID_Table2", "Table2", "ID_Table1 = " & Me.ID_Table1 & " AND Field1= '" & Me.Field1 & "'" & _
" AND ID_Table2 <> " & Nz(Me.ID_Table2, 0)), 0)
But I'm still struggling with micron's solution, can't figure that one out. For now I'll close this thread, but If I figure out how Combobox doesn't show selected Items anymore, I'll post that too.