There are two ways:
1. Clear all the items from Combo box and add all the items again.
vb.net Code:
Private Sub Button2_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs _
) Handles Button2.Click
ComboBox1.Items.Clear()
ComboBox1.Items.Add("One")
ComboBox1.Items.Add("Two")
ComboBox1.Items.Add("Three")
ComboBox1.Items.Add("Four")
End Sub
2. Before adding any item to the combo box, check for it's existence in the combo box item list collection. If not found then add it else ignore.
vb.net Code:
Private Sub Button1_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs _
) Handles Button1.Click
If ComboBox1.Items.IndexOf(TextBox1.Text) = -1 Then
ComboBox1.Items.Add(TextBox1.Text)
End If
End Su