Hello everyone,

I’m working on a customization in Odoo 16 where I added a new field on vendor bills:

bill_merge_id = fields.Many2one(
    "account.move",
    string="Bill Merge",
    domain="[('partner_id', '=', partner_id),
             ('move_type', '=', 'in_invoice'),
             ('company_id', '=', company_id),
             ('state', '!=', 'cancel')]",
)

The idea is:

When the user selects a vendor bill in bill_merge_id, the invoice lines should be replaced with the invoice lines of the selected bill.

Here is my current code:

@api.onchange('bill_merge_id')
    def _onchange_bill_merge_id(self):
        _logger.info("Bill merge onchange triggered")

        # Prevent recursion and duplication on saving/confirm
        self = self.with_context(bill_merge_skip=True)

        if not self.bill_merge_id:
            self.invoice_line_ids = [(5, 0, 0)]
            return

        if self.env.context.get("bill_merge_skip"):
            return

        source_bill = self.bill_merge_id

        line_commands = []
        for line in source_bill.invoice_line_ids:
            line_commands.append((0, 0, {
                'name': line.name,
                'product_id': line.product_id.id,
                'quantity': line.quantity,
                'price_unit': line.price_unit,
                'account_id': line.account_id.id,
            }))

        self.invoice_line_ids = line_commands

The problem

When I select a bill, the invoice lines correctly update.

But when I click Save or Confirm, Odoo duplicates the invoice lines — meaning the same lines get inserted again.

So instead of:

Line A
Line B

I get:

Line A
Line B
Line A
Line B

What I’ve tried

• Clearing the lines with (5, 0, 0)

• Using new_ids = [(0,0,...)] instead of create()

• Adding context flags like "skip_onchange": True

The duplication still happens because Odoo re-triggers onchange during create() and write().

My Question

What is the correct Odoo 16 approach to update invoice lines based on a selected bill without causing duplication during save/confirm?

Should I:

1. Use a button instead of @api.onchange?

2. Use api.depends instead of onchange?

3. Use context flags in override of create() and write()?

4. Something else entirely?

I only want the lines to appear once — exactly when the user selects a bill — and not be duplicated later.

Any guidance or best practices would be greatly appreciated.

Thanks!