Free trial, or $10 for the full version. I'd like to emphasize again that the "duplicate mark" warning happens a LOT, and it's one warning that you can probably just ignore.
If you are on subscription, Autodesk provide a renumbering tool.
Also go the the autodesk exchange store and search the Revit add-ins area. There are several free renumbering tools, some of which come with other useful free productivity tools.
Access a broad range of knowledge to help get the most out of your products and services.