If someone reconnects over and over at the start of the experiment, they get new user records each time, even if their prolific id is already assigned. This happens when someone closes their browser window, then follows the link from Prolific. We should instead look up their existing user record.

@vivtsai 's suggestion: "What do you think about (if the person has their Prolific ID in the URL params and is reconnecting) having a popup: "continue as " or "start over with a new profile" (where clicking the latter closes/boots the old profile)? And then no change for people without prolific IDs."

(expect a PR shortly)