ActiveRecord::RecordInvalid: Validation failed: Email has already been taken

[sentry]

Sentry Issue: HACKWEEK-6C

ActiveRecord::RecordInvalid: Validation failed: Email has already been taken
  app/models/project.rb:110:in `join!'
    users << user
  app/controllers/projects_controller.rb:116:in `join'
    if @project.join!(current_user)
...
(109 additional frame(s) were not displayed)