How to Indicate the Unique Customer ID that not repeated?

I have a set of data and I want to create a calculated field in ADODB query that only states a 'unique' against the customer ID that is not repeated more than once.

Would that wok in select statement?

IIF(COUNT T1.[Customer ID] <> 1 ,Null ,'Unique') as [Unique]

If you prefer to get the result with ADODB, please try.

Option Explicit
Sub ExtractUniqueCustomer()
    Dim conn As Object
    Dim rs As Object
    Dim strSql As String
    Set conn = CreateObject("ADODB.Connection")
    conn.Open "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & ThisWorkbook.FullName & _
        ";Extended Properties=""Excel 12.0;HDR=Yes;IMEX=1"";"
    Set rs = CreateObject("ADODB.Recordset")
    strSql = " SELECT T1.[Invoice], T1.[Customer ID], IIF(T2.[cnt]=1,'Unique','') FROM [Sheet2$] T1 LEFT JOIN " & _
        " (SELECT [Customer ID], COUNT(1) AS cnt FROM [Sheet2$] GROUP BY [Customer ID] HAVING COUNT(1) = 1) T2 " & _
        " ON T1.[Customer ID]=T2.[Customer ID]"
    rs.Open strSql, conn
    ActiveSheet.Cells(2, 1).CopyFromRecordset rs
    rs.Close
    Set rs = Nothing
    conn.Close
    Set conn = Nothing
End Sub