diff --git a/addons/purchase/models/account_invoice.py b/addons/purchase/models/account_invoice.py
index 1391e5f0409..9ca38fde243 100644
--- a/addons/purchase/models/account_invoice.py
+++ b/addons/purchase/models/account_invoice.py
@@ -15,6 +15,13 @@ class AccountMove(models.Model):
         states={'draft': [('readonly', False)]},
         string='Purchase Order',
         help="Auto-complete from a past purchase order.")
+    
+    def _get_invoice_reference(self):
+        self.ensure_one()
+        vendor_refs = [ref for ref in set(self.line_ids.mapped('purchase_line_id.order_id.partner_ref')) if ref]
+        if self.ref:
+            return [ref for ref in self.ref.split(', ') if ref and ref not in vendor_refs] + vendor_refs
+        return vendor_refs
 
     @api.onchange('purchase_vendor_bill_id', 'purchase_id')
     def _onchange_purchase_auto_complete(self):
@@ -55,9 +62,8 @@ class AccountMove(models.Model):
         self.invoice_origin = ','.join(list(origins))
 
         # Compute ref.
-        refs = set(self.line_ids.mapped('purchase_line_id.order_id.partner_ref'))
-        refs = [ref for ref in refs if ref]
-        self.ref = ','.join(refs)
+        refs = self._get_invoice_reference()
+        self.ref = ', '.join(refs)
 
         # Compute payment_reference.
         if len(refs) == 1: