Incorrect "Someone has already registered this as a deploy key" message.

I had an SSH key I added mistakenly as a deployment key (I didn't realize that that would be read-only). So I fixed that by removing it from the list of deployment keys, but now when I try to add that key to my account, I get the error "Someone has already registered this as a deploy key."

How can I clear this out so it will let me add the key?