A double booking is when the diary holds two appointments that cannot both happen. Usually it is a mistake: two people took bookings from two places, one in the paper book and one on a phone, or someone booked a client into a gap that was really a colour still processing. Occasionally it is deliberate. A stylist books a quick cut during another client's processing time, which works as long as the timing is planned rather than hoped for.
The accidental kind costs more than the slot. One client waits or is turned away, and the one turned away is the one most likely to write a review about it.
In practice
A nail salon takes bookings at the desk, by social media message and by phone. In one month it has 3 double bookings, all from social media messages answered on a phone while the desk diary was being changed. The fix was a rule rather than new software: no time is offered to anyone until it is written in the one diary, and every source, including messages, is treated as a request until then.
When it happens anyway, decide on the spot who keeps the slot, usually whoever was confirmed first. Tell the other client honestly and offer something worth having: the next good time, and a small discount or an add-on at no charge.
Keeping one diary and leaving buffer time between appointments are the two habits that prevent most double bookings.
In Glow Line
Glow Line does not write in your diary, so it cannot double-book you. Calls and the booking page produce requests, and only you, with the diary open, turn them into bookings. See how bookings work.