Django form: ask for confirmation before committing to db

Update: The solution can be found as a separate answer

I am making a Django form to allow users to add tvshows to my db. To do this I have a Tvshow model, a TvshowModelForm and I use the generic class-based views CreateTvshowView/UpdateTvshowView to generate the form.

Now comes my problem: lets say a user wants to add a show to the db, e.g. Game of Thrones. If a show by this title already exists, I want to prompt the user for confirmation that this is indeed a different show than the one in the db, and if no similar show exists I want to commit it to the db. How do I best handle this confirmation?

Some of my experiments are shown in the code below, but maybe I am going about this the wrong way. The base of my solution is to include a hidden field force, which should be set to 1 if the user gets prompted if he is sure he wants to commit this data, so that I can read out whether this thing is 1 to decide whether the user clicked submit again, thereby telling me that he wants to store it.

I would love to hear what you guy's think on how to solve this.

views.py

class TvshowModelForm(forms.ModelForm):
    force = forms.CharField(required=False, initial=0)
    def __init__(self, *args, **kwargs):
        super(TvshowModelForm, self).__init__(*args, **kwargs)

    class Meta:
        model = Tvshow
        exclude = ('user')

class UpdateTvshowView(UpdateView):
    form_class = TvshowModelForm
    model = Tvshow
    template_name = "tvshow_form.html"

    #Only the user who added it should be allowed to edit
    def form_valid(self, form):
        self.object = form.save(commit=False)
        #Check for duplicates and similar results, raise an error/warning if one is found     
        dup_list = get_object_duplicates(Tvshow, title = self.object.title)
        if dup_list:
            messages.add_message(self.request, messages.WARNING, 
'A tv show with this name already exists. Are you sure this is not the same one? Click submit again once you\'re sure this is new content'
               )
#            Experiment 1, I don't know why this doesn't work
#            form.fields['force'] = forms.CharField(required=False, initial=1)

#            Experiment 2, does not work: cleaned_data is not used to generate the new form
#            if form.is_valid():
#                form.cleaned_data['force'] = 1

#            Experiment 3, does not work: querydict is immutable
#            form.data['force'] = u'1'

        if self.object.user != self.request.user:
            messages.add_message(self.request, messages.ERROR, 'Only the user who added this content is allowed to edit it.')

        if not messages.get_messages(self.request):
            return super(UpdateTvshowView, self).form_valid(form)
        else:
            return super(UpdateTvshowView, self).form_invalid(form)