Imports Excel = Microsoft.Office.Interop.Excel
Public Class Form1
    Dim App As New Excel.Application
    Dim Book As Excel.Workbook
    Dim Sh1 As Excel.Worksheet
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim s As String
        Dim x As Long
        Book = App.Workbooks.Open(Application.StartupPath & "\Book.xlsx")
        Sh1 = Book.Worksheets("Лист1")
        s = Sh1.UsedRange.Rows.Count
        With Sh1
            Do Until x = s
                x = x + 1
                ' тут нужно создать проверку есть ли такой элемент в списке или нет
               ' если нет то:
               ComboBox1.Items.Add(.Range("A" & x).Value2)
            Loop
        End With
        Book.Close()
    End Sub
End Class