Data with Mix of Unique and Duplicate IDs - Create new column that creates Unique IDs for all

Let's say your IDs are in D2 and down.

A formula solution:

Enter the following formula in E2 or another cell in row 2:

=D2&"-"&COUNTIF(D$2:D2,D2)

Fill down.

A macro solution:

Sub CreateUniqueIDs()
    Const col = 4 ' column D
    Dim m As Long
    Application.ScreenUpdating = False
    Cells(1, col + 1).EntireColumn.Insert
    m = Cells(Rows.Count, col).End(xlUp).Row
    With Range(Cells(2, col + 1), Cells(m, col + 1))
        .FormulaR1C1 = "=RC[-1]&""-""&COUNTIF(R2C[-1]:RC[-1],RC[-1])"
        .NumberFormat = "@"
        .Value = .Value
    End With
    Application.ScreenUpdating = True
End Sub