diff --git a/mrp_lot_number_propagation/models/mrp_production.py b/mrp_lot_number_propagation/models/mrp_production.py
index 40ea6e6fd..8d8b090d4 100644
--- a/mrp_lot_number_propagation/models/mrp_production.py
+++ b/mrp_lot_number_propagation/models/mrp_production.py
@@ -111,13 +111,30 @@ class MrpProduction(models.Model):
                 and m.state not in ("done", "cancel")
             )
             if finish_moves and not finish_moves.quantity_done:
-                lot = self.env["stock.production.lot"].create(
-                    {
-                        "product_id": order.product_id.id,
-                        "company_id": order.company_id.id,
-                        "name": order.propagated_lot_producing,
-                    }
+                lot_model = self.env["stock.production.lot"]
+                lot = lot_model.search(
+                    [
+                        ("product_id", "=", order.product_id.id),
+                        ("company_id", "=", order.company_id.id),
+                        ("name", "=", order.propagated_lot_producing),
+                    ],
+                    limit=1,
                 )
+                if lot.quant_ids:
+                    raise UserError(
+                        _(
+                            "Lot/Serial number %s already exists and has been used. "
+                            "Unable to propagate it."
+                        )
+                    )
+                if not lot:
+                    lot = self.env["stock.production.lot"].create(
+                        {
+                            "product_id": order.product_id.id,
+                            "company_id": order.company_id.id,
+                            "name": order.propagated_lot_producing,
+                        }
+                    )
                 order.with_context(lot_propagation=True).lot_producing_id = lot
 
     def write(self, vals):
diff --git a/mrp_lot_number_propagation/tests/test_mrp_production.py b/mrp_lot_number_propagation/tests/test_mrp_production.py
index 90d22ede9..b464e9a44 100644
--- a/mrp_lot_number_propagation/tests/test_mrp_production.py
+++ b/mrp_lot_number_propagation/tests/test_mrp_production.py
@@ -57,6 +57,47 @@ class TestMrpProduction(Common):
         self.order.button_mark_done()
         self.assertEqual(self.order.lot_producing_id.name, self.LOT_NAME)
 
+    def test_order_post_inventory_lot_already_exists_but_not_used(self):
+        self._update_stock_component_qty(self.order)
+        self.order.action_confirm()
+        self._set_qty_done(self.order)
+        self.assertEqual(self.order.propagated_lot_producing, self.LOT_NAME)
+        # Create a lot with the same number for the finished product
+        # without any stock/quants (so not used at all) before validating the MO
+        existing_lot = self.env["stock.production.lot"].create(
+            {
+                "product_id": self.order.product_id.id,
+                "company_id": self.order.company_id.id,
+                "name": self.order.propagated_lot_producing,
+            }
+        )
+        self.order.button_mark_done()
+        self.assertEqual(self.order.lot_producing_id, existing_lot)
+
+    def test_order_post_inventory_lot_already_exists_and_used(self):
+        self._update_stock_component_qty(self.order)
+        self.order.action_confirm()
+        self._set_qty_done(self.order)
+        self.assertEqual(self.order.propagated_lot_producing, self.LOT_NAME)
+        # Create a lot with the same number for the finished product
+        # with some stock/quants (so it is considered as used) before
+        # validating the MO
+        existing_lot = self.env["stock.production.lot"].create(
+            {
+                "product_id": self.order.product_id.id,
+                "company_id": self.order.company_id.id,
+                "name": self.order.propagated_lot_producing,
+            }
+        )
+        self._update_qty_in_location(
+            self.env.ref("stock.stock_location_stock"),
+            self.order.product_id,
+            1,
+            lot=existing_lot,
+        )
+        with self.assertRaisesRegex(UserError, "already exists and has been used"):
+            self.order.button_mark_done()
+
     def test_confirm_with_variant_ok(self):
         self._add_color_and_legs_variants(self.bom_product_template)
         self._add_color_and_legs_variants(self.product_template_tracked_by_sn)