If you have the option toggled to use MYOB invoice sequence number your Invoice numbering sequence is being dictated by your MYOB invoice sequence.
If you do not have the option "Warn for Duplicate" ticked under the sales tab in MYOB Accountright, (see Setup-->Preferences) then MYOB will allow duplicate invoice numbers.
MYOB does not prevent duplicate invoice numbers and a user can overwrite the invoice number when generating a invoice in MYOB, which creates a new numbering sequence. So when you send an invoice from Tradify after this, MYOB will assign a number that is already in use. With the option above ticked, MYOB will look for the first available number in the current numbering sequence, so duplicates are avoided. This will not prevent a user from creating a duplicate manually in MYOB and overriding the warning.