Expanded the fix in [12663] to cover CSS declarations, which were also...

    def add_css(self, data):

        if data:

            for medium, paths in data.items():

                self._css.setdefault(medium, []).extend([path for path in paths if path not in self._css[medium]])

                for path in paths:

                    if not self._css.get(medium) or path not in self._css[medium]:

                        self._css.setdefault(medium, []).append(path)

    def __add__(self, other):

        combined = Media()

<script type="text/javascript" src="http://media.other.com/path/to/js2"></script>

<script type="text/javascript" src="https://secure.other.com/path/to/js3"></script>

# Regression check for #12879: specifying the same JS file multiple

# times in a single Media instance should result in that file only

# being included once.

# Regression check for #12879: specifying the same CSS or JS file

# multiple times in a single Media instance should result in that file

# only being included once.

>>> class MyWidget4(TextInput):

...     class Media:

...         css = {'all': ('/path/to/css1', '/path/to/css1')}

...         js = ('/path/to/js1', '/path/to/js1')

>>> w4 = MyWidget4()

>>> print w4.media

<link href="/path/to/css1" type="text/css" media="all" rel="stylesheet" />

<script type="text/javascript" src="/path/to/js1"></script>