What causes On Error Resume Next to be ignored ??

I have a piece of code that exploits the Collection limit of unique key
values only, however it still throws an error when trying to add a duplicate
key to the Collection - as if the On Error Resume Next is being ignored

Dim NoDupes As New Collection
Dim AllCells As Range, Cell As Range
Set AllCells = Range("A1:A100")

' The next statement ignores the error caused by attempting to add a
duplicate key to the collection.
On Error Resume Next
For Each Cell In AllCells
NoDupes.Add Cell.Value, CStr(Cell.Value)
Next Cell
' Resume normal error handling
On Error GoTo 0

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Cheers
Nigel