[FIX] web: field wrongly considered as always invisible

If a field is present twice in a view, the field information
stored in the relational model `activeFields` holds and consider
only the latest xml node found.

Therefore, a field is sometimes considered alwaysInvisible
when it is visible and editable.  This means that you might click
on the 'Save' button and get a server error because a required field
is not set when you should the required field should have been
highlighted and requested before making a `create` rpc request.

This commit makes sure that for the alwaysInvisible logic,
both nodes are considered.

closes odoo/odoo#110924

Signed-off-by: Georis François (fge) <fge@odoo.com>