There was another thread on double depositing, but it was closed because common app clearly states double depositing is not allowed.
While I doubt that schools will rescind offers this year for double depositing, your GC won’t send a final transcript to two schools. Your student might not be able to participate in certain processes without submission of final transcript…things like choosing housing, taking placement tests, registering for courses might all be prohibited.
Seems like you can still do the plan happymom laid out in #8. There are never two deposits outstanding, it’s just like in prior years if you deposit at another school by the May 1 deadline, then get off a waitlist later, you accept the waitlist offer and tell school #1 you aren’t coming (likely losing your deposit in the process).