Since you will get (1062, "Duplicate entry for key 'friendship_friendshiprequest_from_user_id_") you should add some validation for that. I made a simple view to show you an example.

views.py

@login_required
def friendship_add_friend(request, user_id, user_name):
    to_user = User.objects.get(pk=user_id)
    from_user = request.user
    if request.user == to_user:
        messages.error(request, (
            'You can not add yourself as a friend...')
        )
    else:
        try: 
            already_friends = Friend.objects.get(from_user=from_user, to_user=to_user)
        except:
            already_friends = None
        if already_friends:
            messages.error(request, (
                u'You are already friends with %s' % slugify(user_name))
            )
        else:
            try:
                created, sent_request = FriendshipRequest.objects.get_or_create(from_user=from_user, to_user=to_user)
            except:
                sent_request = None

            if sent_request:
                messages.success(request, (
                        u'Your friend request has been sent to %s! '
                        u'You must now wait for %s to accept.' % (slugify(user_name), slugify(user_name)))
                )
            else:
                messages.error(request, (
                    u'You have already sent a friend request to %s...' % slugify(user_name))
                )
    
    return redirect(reverse('friendship_view_friends', args=(user_id, slugify(user_name))))