Flatten a level of sublists before checking for duplicate fields.
When given sublists such as:
```python
class FooAdmin(admin.ModelAdmin):
fields = ('one', ('one', 'two'))
```
The previous code did not correctly detect the duplicated 'one' field.
Thanks to jwa for the report.
There was an inconsistency between how the label_tag for forms were
generated depending on which method was used: as_p, as_ul and as_table
contained code to append the label_suffix where as label_tag called on a
form field directly did NOT append the label_suffix. The code for
appending the label_suffix has been moved in to the label_tag code of
the field and the HTML generation code for as_p, as_ul and as_table now
calls this code as well.
This is a backwards incompatible change because users who have added the
label_suffix manually in their templates may now get double label_suffix
characters in their forms.
