When I sync my recipes from MasterCook on the computer to MasterCook.com, the categories are duplicated.
The developers are working on this issue. It's related to the default online categories. I would suggest you do not use them for now until our developers fix this bug. They are these ones:
Instead make your own. Open your cookbook in MasterCook on the computer. Select Modify > Categories from the Tools menu. Rename Appetizer to Appetizers with an s. Select the Appetizer category. Type in its new name. Press the RENAME button.
Since that is not a default online category, it won't be duplicated when you sync to online.