Howdy folks. Jeff here, back from my summer holiday in the Coromandel Peninsula in the North Island of New Zealand, where I’ve been staring at this for the last 21 days:
DDOE_Identifying duplicates between lists_Opoutere

For the next 344 I’ll be staring at this:
DDOE_Identifying duplicates between lists_Excel
God, it’s good to be home.

A while back I answered this thread for someone wanting to identify any duplicate values found between 4 separate lists.

The way I understood the question, if something appears in each of the four lists, the Op wanted to know about it. If an item just appeared in 3 lists but not all 4, then they didn’t want it to be picked up. And the lists themselves might have duplicates within each list.

Say we’ve got these 4 lists:
DDOE_Identifying duplicates between lists_Names

We can’t simply use Conditional Formatting, because that will include duplicate names that don’t appear in each and every column, such as ‘Mike’:
DDOE_Identifying duplicates between lists_Wrong

Rather, we only want names that appear in every column:
DDOE_Identifying duplicates between lists_Right

I wrote a routine that handled any number of lists, using two dictionaries and a bit of shuffling between them. And the routine allows users to select either a contiguous range if their lists are all in one block, or multiple non-contiguous ranges if they aren’t.

  1. The user gets prompted for the range where they want the identified duplicates to appear:
    DDOE_Identifying duplicates between lists_Select Output Range

  3. Then they get prompted to select the first list. The items within that list get added to Dic_A. (If they select more than one columns, the following steps get executed automatically).
    DDOE_Identifying duplicates between lists_Select First Range

  5. Next they get prompted to select the 2nd list, at which point the code attempts to add each new item to Dic_A. If an item already exists in Dic_A then we know it’s a duplicate between lists, and so we add it to Dic_B. At the end of this, we clear Dic_A. Notice that any reference to selecting a contiguous range has been dropped from the InputBox:
    DDOE_Identifying duplicates between lists_Select 2nd range

  7. When they select the 3rd list, then it attempts to add each new item to Dic_B, and if an error occurs, then we know it’s a duplicate between lists, and so we add it to Dic_A. At the end of this, we clear Dic_B. We carry on in this manner until the user pushes Cancel (and notice now that the InputBox message tells them to push cancel when they’re done):
    DDOE_Identifying duplicates between lists_Select 3rd range

Pretty simple: just one input box, an intentional infinite loop, and two dictionaries that take turns holding the current list of dictionaries. Hours of fun.

Only problem is, I had forgotten to account for the fact that there might be duplicates within a list. The old code would have misinterpreted these duplicates as between-list duplicates, rather than within-list duplicates. The Op is probably completely unaware, and probably regularly bets the entire future of his country’s economy based on my bad code. Oops.

I’ve subsequently added another step where a 3rd dictionary is used to dedup the items in the list currently being processed. Here’s the revised code. My favorite line is the Do Until “Hell” = “Freezes Over” one.

Sub DuplicatesBetweenLists()

    Dim rngOutput As Range
    Dim dic_A As Object
    Dim dic_B As Object
    Dim dic_Output As Object
    Dim lng As Long
    Dim lngRange As Long
    Dim varItems As Variant
    Dim strMessage As String

    varItems = False
    On Error Resume Next
    Set varItems = Application.InputBox _
                    (Title:="Select Output cell", _
                     Prompt:="Where do you want the duplicates to be output?", Type:=8)
    If Err.Number = 0 Then ‘user didn’t push cancel
        On Error GoTo 0
        Set rngOutput = varItems
        Set dic_A = CreateObject("Scripting.Dictionary")
        Set dic_B = CreateObject("Scripting.Dictionary")
        Set dic_Output = CreateObject("Scripting.Dictionary")
        lngRange = 1
        Do Until "Hell" = "Freezes Over"    ‘We only want to exit the loop once the user pushes Cancel,
                                            ‘ or if their initial selection was a 2D range
            Select Case lngRange
                Case 1: strMessage = vbNewLine & vbNewLine & "If your ranges form a contiguous block (i.e. the ranges are side-by-side), select the entire block."
                Case 2: strMessage = ""
                Case Else: strMessage = vbNewLine & vbNewLine & "If you have no more ranges to add, push Cancel"
            End Select
            varItems = Application.InputBox(Title:="Select " & lngRange & OrdinalSuffix(lngRange) & " range…", _
                                                Prompt:="Select the " & lngRange & OrdinalSuffix(lngRange) & " range that you want to process." & strMessage, _
            If VarType(varItems) = vbBoolean Then
                lngRange = lngRange – 1
                If lngRange = 0 Then GoTo errhandler:
                Exit Do
                DuplicatesBetweenLists_AddToDictionary varItems, lngRange, dic_A, dic_B
                If UBound(varItems, 2) > 1 Then
                    lngRange = lngRange – 1
                    Exit Do ‘Data is in a contigous block
                End If
            End If
        ‘Write any duplicate items back to the worksheet.
        If lngRange Mod 2 = 0 Then
            Set dic_Output = dic_B
        Else: Set dic_Output = dic_A
        End If
        If dic_Output.Count > 0 Then
            If dic_Output.Count < 65537 Then
                rngOutput.Resize(dic_Output.Count) = Application.Transpose(dic_Output.Items)
                ‘The dictionary is too big to transfer to the workheet
                ‘ because Application.Transfer can’t handle more than 65536 items.
                ‘ So we’ll transfer it to an appropriately oriented variant array,
                ‘ then transfer that array to the worksheet WITHOUT application.transpose
                ReDim varOutput(1 To dic_Output.Count, 1 To 1)
                For lng = 1 To dic_Output.Count
                    varOutput(lng, 1) = dic_Output.Item(lng)
                Next lng
                rngOutput.Resize(dic_Output.Count) = varOutput
            End If ‘If dic_Output.Count < 65537 Then
            MsgBox "There were no numbers common to all " & lngRange & " columns."
        End If ‘If dic_Output.Count > 0 Then
            End If ‘If VarType(varItems) <> vbBoolean Then ‘User didn’t cancel

        Set dic_A = Nothing
        Set dic_B = Nothing
        Set dic_Output = Nothing

End Sub

Private Function DuplicatesBetweenLists_AddToDictionary(varItems As Variant, ByRef lngRange As Long, ByVal dic_A As Object, ByVal dic_B As Object)
    Dim lng As Long
    Dim dic_dedup As Object
    Dim varItem As Variant
    Dim lPass As Long
    Set dic_dedup = CreateObject("Scripting.Dictionary")

    For lPass = 1 To UBound(varItems, 2)

        If lngRange = 1 Then
            ‘First Pass: Just add the items to dic_A
            For lng = 1 To UBound(varItems)
                If Not dic_A.exists(varItems(lng, 1)) Then dic_A.Add varItems(lng, 1), varItems(lng, 1)
            ‘ Add items from current pass to dic_Dedup so we can get rid of any duplicates within the column.
            ‘ Without this step, the code further below would think that intra-column duplicates were in fact
            ‘ duplicates ACROSS the columns processed to date
            For lng = 1 To UBound(varItems)
                If Not dic_dedup.exists(varItems(lng, lPass)) Then dic_dedup.Add varItems(lng, lPass), varItems(lng, lPass)
             ‘Find out which Dictionary currently contains our identified duplicate.
             ‘ This changes with each pass.
             ‘   *  On the first pass, we add the first list to dic_A
             ‘   *  On the 2nd pass, we attempt to add each new item to dic_A.
             ‘      If an item already exists in dic_A then we know it’s a duplicate
             ‘      between lists, and so we add it to dic_B.
             ‘      When we’ve processed that list, we clear dic_A
             ‘   *  On the 3rd pass, we attempt to add each new item to dic_B,
             ‘      to see if it matches any of the duplicates already identified.
             ‘      If an item already exists in dic_B then we know it’s a duplicate
             ‘      across all the lists we’ve processed to date, and so we add it to dic_A.
             ‘      When we’ve processed that list, we clear dic_B
             ‘   *  We keep on doing this until the user presses CANCEL.
             If lngRange Mod 2 = 0 Then ‘dic_A currently contains any duplicate items we’ve found in our passes to date
                 ‘Test if item appears in dic_A, and IF SO then add it to dic_B
                 For Each varItem In dic_dedup
                     If dic_A.exists(varItem) Then
                         If Not dic_B.exists(varItem) Then dic_B.Add varItem, varItem
                     End If

             Else ‘dic_B currently contains any duplicate items we’ve found in our passes to date
                 ‘Test if item appear in dic_B, and IF SO then add it to dic_A
                 For Each varItem In dic_dedup
                     If dic_B.exists(varItem) Then
                         If Not dic_A.exists(varItem) Then dic_A.Add varItem, varItem
                     End If
            End If
        End If
        lngRange = lngRange + 1

End Function

Function OrdinalSuffix(ByVal Num As Long) As String
‘Code from

        Dim N As Long
        Const cSfx = "stndrdthththththth" ‘ 2 char suffixes
        N = Num Mod 100
        If ((Abs(N) >= 10) And (Abs(N) <= 19)) _
                Or ((Abs(N) Mod 10) = 0) Then
            OrdinalSuffix = "th"
            OrdinalSuffix = Mid(cSfx, _
                ((Abs(N) Mod 10) * 2) – 1, 2)
        End If
    End Function

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