Thanks Jim & Ron for your feedback and helpful input. I modified this
project as follows...

The caller sub:
--I added an optional notification that asks the user if they want to
run a process on a returned list when the number of matches is
reported. This can be swapped out for the former notification via
'Commenting'. (Code for the process to call needs to be added as
appropriate to user's needs)

The StripDupes() function:
--In the case where no matches are found, the function makes no changes
to the worksheet.
--Iteration of the Collection acts on 'Key'.
--The 2 CStr() functions for adding values to 'Item' were replaced with
'vbNullString'


Final drafts...

Sub CompareCols_StripDupes()
Dim bSuccess As Boolean, lMatchesFound As Long
Dim vAns As Variant, sMsg As String

sMsg = _
"Do you want to remove any duplicate items in the non-matches?" _
& vbLf & vbLf & "(Doing so will return a list of unique items)"
vAns = MsgBox(sMsg, vbYesNo + vbQuestion)

If vAns = vbNo Then
bSuccess = StripDupes(lMatchesFound) '//dupes allowed
Else
bSuccess = StripDupes(lMatchesFound, False) '//no dupes allowed
End If 'vAns = vbNo

If lMatchesFound = 0 Then MsgBox "No matches found!": Exit Sub
If lMatchesFound < 0 Then
sMsg = "Both columns must have more than 1 item!" _
& vbLf & vbLf _
& "Please try again: specify different columns!"
MsgBox sMsg, vbExclamation
Exit Sub
End If 'lMatchesFound < 0

If bSuccess Then
sMsg = Format(CStr(lMatchesFound), "#,##0") _
& " Matches were found"
If vAns = vbYes Then _
sMsg = sMsg & " (including non-match duplicates)"
MsgBox sMsg '//comment out if using option below

'Optional: Ask to run a process on the new list
' sMsg = sMsg & vbLf & vbLf _
' & "Do you want to process the new list?"
'
' vAns = MsgBox(sMsg, vbYesNo + vbQuestion)
' If vAns = vbYes Then
' 'Code... ('Call' a process to act on the new list)
' End If 'vAns = vbYes
Else
MsgBox "An error occured!"
End If 'bSuccess
End Sub

Function StripDupes(Matches As Long, _
Optional AllowDupes As Boolean = True) As Boolean
' Compares 2 user-specified cols and removes matches found.
' User can also specific target col to receive revised list.
'
' Args In: Matches: ByRef var to return number of matches found to
' the caller.
'
' AllowDupes: True by default. Keeps duplicate non-match
' values in col to remove dupes from. If passing False,
' duplicate items in non-match col are removed.
'
' Returns: True if matches found and no error occurs;
' False if a: matches not found --OR-- error occurs;
' b: either input col has less than 2 items.
'
' Sources: Ron Rosenfeld, Jim Cone, GS (Garry Sansom)

Dim i&, j&, lMatchesFound& 'as long
Dim vRngA, vRngB, vRngOut(), vAns 'as variant
Dim sRngOut As String

'Get the label of the columns to act on
Const sMsg As String = "Please enter the label of the column"
'Column to filter
vAns = Application.InputBox(sMsg _
& " to remove duplicates from", Type:=2)
If vAns = False Or vAns = "" Then Exit Function
vRngA = Range(vAns & "1:" & vAns _
& Cells(Rows.Count, vAns).End(xlUp).Row)
sRngOut = vAns '//output goes here unless specified below
'Column to be checked
vAns = Application.InputBox(sMsg _
& " to check for duplicates", Type:=2)
If vAns = False Or vAns = "" Then Exit Function
vRngB = Range(vAns & "1:" & vAns _
& Cells(Rows.Count, vAns).End(xlUp).Row)

'Make sure lists contain more than 1 item
If Not IsArray(vRngA) Or Not IsArray(vRngB) Then _
Matches = -1: Exit Function

'Column to receive the results
vAns = Application.InputBox(sMsg _
& "where the new list is to go" & vbLf _
& "(Leave blank or click 'Cancel' to use column " _
& UCase$(sRngOut) & ")", Type:=2)
If vAns = False Or vAns = "" Then sRngOut = sRngOut _
Else sRngOut = vAns

Debug.Print Now()
Dim cRngB As New Collection
On Error Resume Next
For j = LBound(vRngB) To UBound(vRngB)
cRngB.Add Key:=CStr(vRngB(j, 1)), Item:=vbNullString
Next 'j
Err.Clear: On Error GoTo ErrExit

If AllowDupes Then '//fastest
On Error GoTo MatchFound
For i = LBound(vRngA) To UBound(vRngA)
If cRngB.Key(CStr(vRngA(i, 1))) < "" Then _
vRngA(i, 1) = "": lMatchesFound = lMatchesFound + 1
skipit:
Next 'i

Else '//slowest
On Error GoTo MatchFound
For i = LBound(vRngA) To UBound(vRngA)
cRngB.Add Key:=CStr(vRngA(i, 1)), Item:=vbNullString
Next 'i
End If 'AllowDupes
Err.Clear: On Error GoTo ErrExit

j = 0: ReDim vRngOut(UBound(vRngA) - lMatchesFound, 0)
For i = LBound(vRngA) To UBound(vRngA)
If Not vRngA(i, 1) = "" Then _
vRngOut(j, 0) = vRngA(i, 1): j = j + 1
Next 'i

If lMatchesFound 0 Then '//only write if lMatchesFound 0
Range(sRngOut & ":" & sRngOut).ClearContents
With Range(sRngOut & "1").Resize(UBound(vRngOut), 1)
.Value = vRngOut
.NumberFormat = "0000000000000" '//optional
.EntireColumn.AutoFit '//optional
End With
End If 'lMatchesFound 0

Debug.Print Now()
ErrExit:
Matches = lMatchesFound: StripDupes = (Err = 0)
Exit Function

MatchFound:
If AllowDupes Then Resume skipit
vRngA(i, 1) = "": lMatchesFound = lMatchesFound + 1: Resume Next

End Function 'StripDupes()

Enjoy!

--
Garry

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