Fix #12388: Vehicle::CopyVehicleConfigAndStatistics not releasing unit number (#12389)

src/vehicle.cpp

@@ -2399,6 +2399,15 @@ void Vehicle::ResetRefitCaps()
 	for (Vehicle *v = this; v != nullptr; v = v->Next()) v->refit_cap = v->cargo_cap;
 }
 
+/**
+ * Release the vehicle's unit number.
+ */
+void Vehicle::ReleaseUnitNumber()
+{
+	Company::Get(this->owner)->freeunits[this->type].ReleaseID(this->unitnumber);
+	this->unitnumber = 0;
+}
+
 /**
  * Handle the loading of the vehicle; when not it skips through dummy
  * orders and does nothing in all other cases.