picking_dispatch: duplicating a delivery order will put the new stock move in the duplicated's dispatch

[Guewen Baconnier @ Camptocamp]

If I create a picking A with:
 - move A1
 - move A2

And create a dispatch D with moves:
 - A1
 - A2

And I duplicate the picking A, so I have a picking B with:
 - move B1
 - move B2

Then the dispatch D contains now:
 - A1
 - A2
 - B1
 - B2

But it should contain:
 - A1
 - A2