db.execute() executing only single time

Question

When I enter amount, name, expinc in my html form it works for the first time, i.e. the entered data goes into database tables for the first time but whenever I try to enter data again it doesn't show up in tables. It looks like db.execute function executing not more than one time.

@app.route("/buy", methods=["GET", "POST"])
@login_required
def buy():
    if request.method == "GET":
        return render_template("buy.html")
    else:
        amounts = int(request.form["amount"])
        names = request.form["name"]
        expincs = request.form["expinc"]

        db.execute("INSERT INTO histories (id, amount, name, expinc) \
                    VALUES(:id, :amount, :name, :expinc)", \
                    id=session["user_id"], amount=usd(amounts),\
                    name=names, expinc=expincs)

        db.execute("INSERT INTO portfolio (id, amount, name, expinc) \
                    VALUES(:id, :amount, :name, :expinc)", \
                    id=session["user_id"], amount=usd(amounts), \
                    name=names, expinc=expincs)
        return redirect(url_for("index"))

Answer 1

There is nothing in the posted code that would prevent the db.execute from executing more than one time. One possible explanation is the primary key or a unique index on the tables is preventing duplicates. For example if id is the primary key on histories, it will only allow one row per user id.