RC5 - duplicate key error when adding German file names to collection

I'm adding files to a vbRichClient5 collection and using the file names as keys. Two German file names are causing an error when added to the collection. The error is a duplicate key error (457). The file names are:

Süsskartoffelsuppe.pdf
Süßkartoffelsuppe.pdf

Is this a bug? Is it intentional? Are there properties I need to set on the collection to instruct it to allow Unicode strings as keys?

TIA