Question:  Is it possible to write a macro which would highlight any duplicate values in column A?

Answer:  Let's take a look at an example.

Download Excel spreadsheet (as demonstrated below)

In our spreadsheet, we've set up column A to contain unique values. On this sheet, we've created a button that when clicked will launch a macro. This macro will highlight any duplicate values in column A.

In our example, we've clicked on the button. Now the background color of the duplicates will turn red as follows:

In this example, the same value has been entered in both cells A2 and A5.

You can press Alt-F11 to view the VBA code.

Please note that the LRows variable in this macro is set to 200 indicating that the macro will test the first 200 rows in column A for duplicates. You may need to change this value to accommodate your volume of data.

Macro Code:

The macro code looks like this:

Sub TestForDups()

    Dim LLoop As Integer
    Dim LTestLoop As Integer
    Dim LClearRange As String

    Dim Lrows As Integer
    Dim LRange As String
    Dim LChangedValue As String
    Dim LTestValue As String

    'Test first 200 rows in spreadsheet for uniqueness
    Lrows = 200
    LLoop = 2

    'Clear all flags
    LClearRange = "A2:A" & Lrows
    Range(LClearRange).Interior.ColorIndex = xlNone

    'Check first 200 rows in spreadsheet
    While LLoop <= Lrows
        LChangedValue = "A" & CStr(LLoop)

        If Len(Range(LChangedValue).Value) > 0 Then

            'Test each value for uniqueness
            LTestLoop = 2
            While LTestLoop <= Lrows
                If LLoop <> LTestLoop Then
                    LTestValue = "A" & CStr(LTestLoop)
                    'Value has been duplicated in another cell
                    If Range(LChangedValue).Value = Range(LTestValue).Value Then
                        'Set the background color to red
                        Range(LChangedValue).Interior.ColorIndex = 3
                        Range(LTestValue).Interior.ColorIndex = 3

                    End If

                End If

                LTestLoop = LTestLoop + 1

            Wend

        End If

        LLoop = LLoop + 1
    Wend

End Sub