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'Controls - How to check for duplicates in Combo Box
Option Explicit
 
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Long, _
    ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Declare Function SendMessageByString Lib "user32" Alias "SendMessageA" (ByVal hWnd As Long, _
    ByVal wMsg As Long, ByVal wParam As Long, lParam As String) As Long
Const CB_SHOWDROPDOWN = &H14F
Const CB_FINDSTRINGEXACT = &H158
 
Private Sub Command1_Click()
    With Combo1
        If Not CheckDuplicates(.hWnd, .Text) Then
            .AddItem .Text
        End If
        SendMessage .hWnd, CB_SHOWDROPDOWN, True, ByVal 0&
    End With
End Sub
 
Private Sub Form_Load()
    'Put some items in the combobox
    With Combo1
        .AddItem "Sunday"
        .AddItem "Monday"
        .AddItem "Tuesday"
        .AddItem "Wednesday"
        .AddItem "Thursday"
        .AddItem "Friday"
        .AddItem "Saturday"
        'Provide the default value
        .Text = .List(0)
        'Open the dropdown list
        SendMessage .hWnd, CB_SHOWDROPDOWN, True, ByVal 0&
    End With
End Sub
 
Private Function CheckDuplicates(chwnd As Long, StringText As String) As Boolean
    CheckDuplicates = SendMessageByString(chwnd, CB_FINDSTRINGEXACT, -1, ByVal StringText) > -1
End Function