Mandatory information about my setup:
Have I searched the tickets before posting? Yes
Have I searched the forum before posting? Yes
Have I read the documentation before posting? Yes
Joomla! version: 3.4.3
PHP version: 5.5.17
MySQL version: 5.5.34-32.0-log
Matukio version: 5.1.3
Direct link to the page that shows the issue: Matukio Admin Bookings Page

Description of my issue:
(1)When two simultaneous bookings are made, Matukio increments the invoice number of the first booking with +4, instead of +1. The second booking however gets incremented with +1. See the two attachments for an example.

(2) It seems that the skipped invoice number gets used for a later booking.

(3) Also, the invoice corresponding to the first booking, doesn't get send by email. (Even when the skipped invoice number gets used for a later booking.)

Extra info
Both bookings are automatically done by a website visitor. So the booking is not drawn up in the backend by an administrator.