Crash when setting tree width with a field already saved multiple times
Ref: https://discuss.tryton.org/t/there-is-a-problem-with-new-feature-to-resize-columns-in-view-list-trytond-7-6/8554
If the database has already duplicate records, we should delete the extra and not fail with:
```
Traceback (most recent call last):
File "/site-packages/trytond/wsgi.py", line 97, in dispatch_request
return endpoint(request, **request.view_args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/site-packages/trytond/protocols/dispatcher.py", line 44, in rpc
return methods.get(request.rpc_method, _dispatch)(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/site-packages/trytond/wsgi.py", line 77, in wrapper
return func(request, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/site-packages/trytond/protocols/wrappers.py", line 206, in wrapper
result = func(request, pool, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/site-packages/trytond/protocols/dispatcher.py", line 216, in _dispatch
result = rpc.result(meth(*c_args, **c_kwargs))
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/site-packages/trytond/ir/ui/view.py", line 554, in set_width
tree_width.width = fields.pop(tree_width.field)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'quantity_year'
```
issue