If you're very unlucky and when trying to upload a new extension to the VSCode marketplace, you may get this error: "An item with the same key has already been added."
If you use the website, you may see this (not particularly helpful) error message.
Of course, you really should be using an automated, repeatable process/script to manage your submissions, and if you do that, you'll still get the same result. (I only tried the website after all other attempts to work out what the duplicate key might be.)
Error: Process completed with exit code 1.
Anyway, for me, it turned out that we had content types for both "json" and ".json" defined. (With and without the leading dot.)
Some systems don't mind this, while it turns out the marketplace does.
I hope this helps someone else find a solution to their problems faster than I did. (10 emails, back and forth with support over 11 days!)
