Add new row in csv with Python

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CSV

Python2.7

So I am trying to add a line in my CSV for every post request. However, the first post is fine. The second post suddenly adds 2 rows (the new and the existing one), the third post adds 3 rows etc. How do I fix this?

What I have tried:

def post(self, request, format=None):
        user=MyUser.objects.get(user=request.user)
        movie = Movie.objects.get(movieId=request.POST['movieId'])
        rating = Rating(ratingValue=request.POST['ratingValue'], movie=movie, user=user)
        rating.save()

        with open('data/train.csv', 'a') as csvfile:
            spamwriter = csv.writer(csvfile)
            spamwriter.writerow([user.userID, request.POST['ratingValue'], int(round(time.time(),0))])

        return Response(status=status.HTTP_200_OK)