Failed to run Flask app after modify database model

Good day, I am new to Flask. In the midst of developing my "to do list" app, I wanted to modify detail of "email" column in databases.py.

Below is databases.py:

from flask_sqlalchemy import SQLAlchemy

# Create SQLAlchemy instance
db = SQLAlchemy()


# Define database models

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    first_name = db.Column(db.String(10), unique=False, nullable=False)
    last_name = db.Column(db.String(10), unique=False, nullable=False)
    email = db.Column(db.String(20), unique=True, nullable=False)
    password = db.Column(db.String(12), nullable=True)

My app requires user to login with email and password hence I believe "email" should be unique. So I changed the unique parameter from False to True.

Based on Flask-Migrate documentation, I should execute commands flask db migrate -m "brief description" and flask db upgrade each time the database models change.

However, I encountered error message RuntimeError: A 'SQLAlchemy' instance has already been registered on this Flask app. Import and use that instance instead. when I execute commands flask --debug run / flask db migrate.

Below is app.py:

from flask import Flask, render_template, request, redirect, url_for, flash, get_flashed_messages, session
from email_validator import validate_email, EmailNotValidError
import secrets
from flask_bootstrap import Bootstrap5
from flask_migrate import Migrate
from flask_session import Session

# Create instance for Bootstrap 5
bootstrap = Bootstrap5()

# Create session instance
user_session = Session()

def create_app():
    app = Flask(__name__)

    # Set database location
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///notes.db'

    # configure user session
    app.config['SESSION_TYPE'] = 'sqlalchemy'
    app.config['SESSION_PERMANENT'] = False
    user_session.init_app(app)

    from databases import db, User

    db.init_app(app)

    with app.app_context():
        db.create_all()

    # Database migration
    '''
    Required actions:
    
    1. execute command in terminal for very first time => Flask --debug run
    2. execute command in terminal => flask db init
    3. execute command in terminal => flask db migrate -m "Initial migration"
    4. execute command in terminal => flask db upgrade
    '''
    migrate = Migrate(app, db)
    
    # Flask-WTF
    app.secret_key = secrets.token_hex(8)

    # initiate Bootstrap5 for the app
    bootstrap.init_app(app)

    from forms import new_user, login_user

    @app.route('/register', methods=['GET', 'POST'])
    def register():

        register_user = new_user()

        if register_user.validate_on_submit():

            profile = User(
                first_name = request.form.get('first_name'),
                last_name = request.form.get('last_name'),
                email = request.form.get('email'),
                password = request.form.get('password')
            )

            db.session.add(profile)
            db.session.commit()

            return  redirect(url_for('login'))

        return  render_template('new_user.html', form=register_user)

    @app.route('/')
    @app.route('/login', methods=['GET', 'POST'])
    def login():

        user_login = login_user()

        if user_login.validate_on_submit():

            invalid_email = None
            invalid_password = None
            email_input = request.form.get('email')
            password_input = request.form.get('password')
            checkbox = request.form.get('checkbox')

            user_record = db.session().execute(
                db.select(User).filter_by(email=email_input)
            ).scalar_one_or_none()

            if not user_record:
                invalid_email = f"Invalid email !!"
                return render_template('login.html', form=user_login, invalid_email=invalid_email)
            elif user_record.password != password_input:
                invalid_password = f"Wrong password !!"
                return render_template('login.html', form=user_login, invalid_password=invalid_password)
            else:
                user_session['email'] = email_input if checkbox else None
                return  redirect(url_for('profile', id=user_record.id))

        return  render_template('login.html', form=user_login)
    
    @app.route('/profile/<int:id>')
    def profile(id):

        if user_session.get('email'):
            
            user_record = db.session().execute(
                    db.select(User).filter_by(id=id)
                ).scalar_one_or_none()

            flash(message=f"Welcome back, { user_record.first_name }", category='info')

            return  render_template('user_homepage.html', user=user_record)
        
        return redirect(url_for('login'))

    return app

Appreciete if anyone can point out my mistake.