A driver can hold only one shift per day on the VoiceERP schedule. The schedule builder stops you from adding a second one, and VoiceERP rejects any save that would put the same person on two shifts on the same date.
What counts as a shift for this rule
Every kind of shift counts. That includes station staff shifts, such as an opener or dispatcher shift. A driver who also works as station staff can't have a route shift and a station staff shift on the same day.
The one exception is a shift the driver gave up as VTO. It stays visible on the grid, but it no longer takes up the driver's day, so you can give them a different shift that day.
The messages you'll see
If you assign a driver, or move a shift onto a day where they already have one, the builder shows "This driver already has a shift on this day" and doesn't make the change.
If a save still contains a double booking, it's refused with "A driver is assigned more than one shift on the same day — remove one of their shifts and save again", followed by the date.
If you save station staff shifts and the person already has a route that day, the save doesn't go through, and you'll see "Couldn't save the station staff schedule".
Dragging a shift onto a driver who already works that day doesn't create a double booking. It swaps the two drivers' shifts.
The marketplace, copy week, and templates follow the same rule
Marketplace: a driver can't claim an open shift or accept a swap that would leave them with two shifts on one day. The driver app tells them why. For example, it shows "You're already scheduled on" with the date.
Copying a previous week or applying a schedule template: a driver who already has a shift on a day is skipped for that day. The copy doesn't add a second shift.
What to do instead
There's no double shift or split shift option. If a driver needs to cover more of the day, change their one shift instead of adding another:
Edit shift details on the shift's menu lets you change its type or note, or reassign it to another driver.
Swap with another driver trades this shift with another scheduled driver's shift that same day.
Unassign shift or Remove shift clears the day so you can give the driver a different shift.