Django Forms - getting data from multiple choice fields

I have a form like so:

class MyForm(forms.Form):
    site = forms.ChoiceField(choices=SITE_CHOICES, label=ugettext_lazy('Site'),)  
    ...
    params = forms.MultipleChoiceField(
             choices=PARAM_CHOICES,
             label=ugettext_lazy('Select Parameters'),
             widget=forms.CheckboxSelectMultiple()
             )

And in my template:

<form action="{% url results %}" method="get">{% csrf_token %}
{% for field in myform %}
<div class="field_wrapper">
    {{ field.errors }}
    {{ field.label_tag }} {{ field }}
</div>
{% endfor %}
<input type="submit" name="submit" value="{% trans 'Query' %}"  />
</form>

My problem is that when I submit the form as GET the variables are like the following:

    site=1&params=foo&params=bar&params=something&submit=Query

my params variable is clearly being overwritten by the last choice? How can get access to the submitted data as separate variables?

Any help appreciated.