TheRecipeManager doesn't have a specific command to remove duplicate recipes. The reason is that you can have multiple variations of the same recipe, with the same name. However, it is easy to identify duplicate recipes and remove them individually.

The default setting for the recipe list is to sort recipes by their name. So you can move down the list looking for duplicates and remove them as they occur. You can click on any of the headings on the recipe list, to order the list by that field.