Why Booking Calendar double-books slots in Google Forms™

Most likely a race condition: the add-on checks whether a slot is open and writes the new booking as two separate steps. If two submissions land close enough together, both checks can see an open slot before either write finishes, so both go through even though the slot should have closed after the first one. The fix is to make the check-and-write a single atomic step, which is how Slot Booking for Forms™ enforces capacity.

A Google Form's dropdown can hold up to 250 choices. Slot Booking for Forms™ uses that full range: it shows the soonest times first, and as those fill or pass, the next times waiting in line take their place automatically. Nothing is capped at a fixed number.

Not if capacity is enforced atomically at the moment of submission. A manual approval queue is a workaround for an add-on that can't guarantee capacity on its own. Slot Booking for Forms™ enforces the limit at submit time, so there's nothing left to approve after the fact.

Why the Accepting responses switch can't do this →

Yes. Slot Booking for Forms™ is a free add-on that does the same job: it turns a Google Form into a booking form with real capacity per slot. The difference is in how bookings are counted. Capacity is checked and claimed in one step at the moment of submission, so a full slot closes itself and the same spot can't be given out twice. It's free in early release, with no card and no account to create.

They're public and unedited, quoted from Booking Calendar's own listing on the Google Workspace Marketplace, dated as shown. Slot Booking for Forms™ is an independent add-on made by Marinette and isn't affiliated with Booking Calendar or its developer, Apps Vietutd.