bench: cron page should give error for crons added with the same ID

Currently it seems that if we add a cron with the same name (ID) as another cron, it will just silently overwrite the existing cron. Instead it should give an error, something like "cron IDs must be unique".

No files or tests are named. Start by locating the cron page and the path that adds crons, then reproduce adding two crons with the same ID; done means the second addition returns an explicit error such as "cron IDs must be unique" instead of silently overwriting the first.

Written by the indexing model from the issue text.