Exhaust Problem (driving me mad)

[Watto]

I recently replaced the section of exhaust that runs from the manifold to the rear section on my 208 , but no matter how tight do up the two bolts that contect the exhaust to the manifold after a few miles they loosen , I've even tried locking the nuts up with another nut underneath them but they still loosen . Help please cos this is really getting to me !!!!