The following will refer to class names, while I've tried my best to ensure that they are self-explanatory, I may be wrong. Please view https://gist.github.com/jamierocks/e10effbc064b3348f2b11142e4fab4c1 to see mappings used.

The issue exists within the class CommandListProvider (fi), and the faulty implementation of DataProvider#getName() (fg#b()).

The current implementation is as follows:

    public String getName() {
        return "Item List";
    }

The corrected implementation may be as follows:

    public String getName() {
        return "Command List";
    }

Of course, other names (such as "Command Data", or "Command Tree") may be more appropriate.

This issue can be seen (not in code) by the following:

jamie@Jamies-Mac-mini  ~/proj/Shroud/run   develop  java -cp server.jar net.minecraft.data.Main --reports
[12:52:20] [main/INFO]: Starting provider: Block List
[12:52:20] [main/INFO]: Block List finished after 201 ms
[12:52:20] [main/INFO]: Starting provider: Item List
[12:52:20] [main/INFO]: Item List finished after 6 ms
[12:52:20] [main/INFO]: Starting provider: Item List
[12:52:21] [main/INFO]: Loaded 0 recipes
[12:52:21] [main/INFO]: Loaded 0 advancements
[12:52:21] [main/INFO]: Item List finished after 693 ms

NOTE: "Item List" appearing twice.