If you invoice from both MYOB and WMS, It creates duplicates invoice numbers because MYOB doesn’t know which invoice numbers to use.
There are two ways to work around in this situation,
Create non-inventory items in MYOB for services and do all the invoices in WMS.
When you create invoices in WMS, you can setup system to put suffix after WMS invoice number.
Login to WMS as Superuser.
Go to Tools > Preferences.
In the Preferences window, click Sales tab.
In this section, tick checkbox beside Use Auto# with:
You can put any characters where “W” in the picture.
Once you setup above settings in WMS, all the invoices created from WMS place W after every invoice number.