Someone else pointed out to me that it can be hard to avoid duplicating work if you don't know that someone else is already working on a ticket. Surprise! We also have a way to deal with that. If you start work on a JIRA ticket, change the status to IN PROGRESS (by hitting the "start work" button).
If it's a new project, submit a JIRA and do the same before you start on it. This also helps new contributors find work to do that isn't already in progress.
