Python script to add data to postgres docker container runs multiple times

I'm trying to find a good way to populate a database with initial data for a simple application. I'm using a tutorial from realpython.com as a starting point. I then run a simple python script after the database is created to add a single entry, but when I do this the data is added multiple times even though I only call the script once. result

population script (test.py):

   from app import db                                                                                                                                                          
   from models import *                                                                                                                                                        

   t = Post("Hello 3")                                                                                                                                                         
   db.session.add(t)                                                                                                                                                           
   db.session.commit()  

edit:

Here is the docker-compose file which i use to build the project:

web:
  restart: always
  build: ./web
  expose:
    - "8000"
  links:
    - postgres:postgres
  volumes:
    - /usr/src/app/static
  env_file: .env
  command: /usr/local/bin/gunicorn -w 2 -b :8000 app:app

nginx:
  restart: always
  build: ./nginx/
  ports:
    - "80:80"
  volumes:
    - /www/static
  volumes_from:
    - web
  links:
    - web:web

data:
  restart: always
  image: postgres:latest
  volumes:
    - /var/lib/postgresql
  command: "true"

postgres:
  restart: always
  image: postgres:latest
  volumes_from:
    - data
  ports:
    - "5432:5432"

it references two different Dockerfiles:

Dockerfile #1 which builds the App container and is 1 line:

FROM python:3.4-onbuild

Dockerfile #2 is used to build the nginx container

FROM tutum/nginx
RUN rm /etc/nginx/sites-enabled/default
ADD sites-enabled/ /etc/nginx/sites-enabled

edit2:

Some people have suggested that the data was persisting over several runs, and that was my initial thought as well. This is not the case, as I remove all active docker containers via docker rm before testing. Also the number of "extra" data is not consistent, ranging randomly from 3-6 in the few tests that I have run so far.