As i wrote in Peter's comment, the problem was in the combo items duplication, that was behaving as something like "sharing items". I replace the duplication line for this ugliest and longer code, and the problem was solved. Hope it helps.
' Clear the items in the current combo,and fills with the ones in a dummy combo
ControlCombo.Items.Clear()
' THIS LINE HAS BEEN COMMENTED AND REPLACED FOR THE FOLLOWING CODE
' ControlCombo.Items.AddRange(ComboPatron.Items.OfType(Of ListItem)().ToArray())
Dim LSource As ListItem
Dim LDestination As ListItem
For i = 0 To ComboPatron.Items.Count - 1
LSource = ComboPatron.Items(i)
LDestination = New ListItem(LSource.Text, LSource.Value)
ControlCombo.Items.Add(LDestination)
Next