[Python] Return error code and message to API caller

I have a web application that put some data into a database using the SQLAlchemy: if the element does not exist into the table it is added else it doesn't. In both cases it shows a popup message to the user telling if the operation was successful or not using the flash command, so it works fine.

The problem is that if I do a POST HTTP request to that service using an external device (for example an Android app or a separate Python script) the response returns always status code 200 even if the record exists into the table.

What I would like to do is:

1. The user add the record
2. If the record does not exists it is added into the database and redirect me to the homepage of the website /home
3. If the record exists return an error code/message to the user so that I can manage that case on client side and redirect to the homepage of the website /home

Code of the python flask:

# creating of the model
class Product(db.Model):
id = db.Column(db.String(100), primary_key=True)
total = db.Column(db.Integer())


# home route
@app.route('/')
def home():
product = Product.query.all()
return render_template('index.html', products=product)


# route for adding todos
@app.route('/add', methods=['POST'])
def add():
if request.is_json:
data = request.get_json()
else:
data = request.form

product = Product(id=data['product_id'], total=data['product_quantity'])
product_exist = Product.query.filter_by(id=product.id).first()

if product_exist:
flash(f"{product.id} already in list", category="error")
else:
db.session.add(product)
db.session.commit()
flash(f"{product.total} {product.id} added successfully.", category="success")

return redirect(url_for('home'))

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