Many Thanks Yaroslav! This is exactly what i was looking for and it works just the way i need it to.

I´m sorry if i was a bit short in my question about the use case. We work with services and each order line on a sales order creates a task in project. Mainly the communication with the customer is made from sales, but the information needs to pass over to task department.
It is not a problem that the complete message history are tracked in each of the corresponding tasks, it still serves its purpose. I understand what you mean with the followers getting duplicates and maybe i´ll have to handle this further on.

What if i want to do the opposite, pass information from task to sales order? Would it be possible to merge tasks messages into one sales order? So that when a sales person communicates with a customer, they also have a complete service history from our technicians.

Many thanks and regards
Peter

Peter, I'm glad the automatic action worked the way you wanted.
But I don't like your second idea, because you want to trigger automatic message creation in the Task, which should trigger automatic message creation in the Sales Order and so on infinitely.
This is not necessarily a blocker in general, for example you can check if the message is created by an actual user and not the system itself ( if record.create_uid.id != 1: ):

if record.model == 'sale.order':
sale_order = env['sale.order'].browse(record.res_id)
if record.create_uid.id != 1:
for task in env['project.task'].search([('sale_line_id', 'in', sale_order.order_line.ids)]):
task.message_post(body=record.body)

but I advise you to contact Odoo partners for more complex solutions.

I understand what you are saying.

I have been looking for some apps that could achieve this more advanced solution but without success. I might leave it as a one way communication channel for the moment and bring it up with my Odoo partner later on.

Your solution is beautiful and of great help to me! Many thanks Yaroslav!

//Peter