Jinja2模板错误：block 'title'被定义两次，如何解决？

Question

jinja2.exceptions.TemplateAssertionError: block 'title' defined twice

{% block content %}
<div class="container">
    {% for message in get_flashed_messages() %}
    <div class="alert alert-warning">
        <button type="button" class="close" data-
dismiss="alert">&times;</button>
        {{ message }}
    </div>
    {% endfor %}
    {% block page_content %}{% endblock %}
</div>
{% endblock %}
##以上是base1.html的内容
======================================
{% extends "base1.html" %} 
{% import "bootstrap/wtf.html" as wtf %} 

{% block page_content %}
<div class="page-header">
    <h1>Hello, {% if name %}{{ name }}{% else %}Stranger{% endif %}!</h1>
</div>
{{ wtf.quick_form(form) }}
{% endblock %}
以上是index1.html的内容

=================主程序=======================
from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField
from wtforms.validators import DataRequired
from flask import Flask, render_template, session, redirect,url_for
# from collections.abc import Sequence
# import _collections_abc
# from typing import Any, Mapping

app=Flask(__name__)
# bootstrap=Bootstrap(app)
app.config["SECRET_KEY"] = "123456"

class NameForm(FlaskForm):
    name = StringField('What is your name?', validators=[DataRequired()])
    submit = SubmitField('Submit')

@app.route("/",methods=["GET", "POST"])
def index():
    name=None
    form = NameForm()
    if form.validate_on_submit(): 
        name = form.name.data
        form.name.data = ''
    return render_template('index1.html', form=form, name=name)
    #     old_name=session.get("name")
    #     if old_name is not None and old_name !=form.name.date: 
    #         flash("看看你输入的什么")
    #     session["name"] = form_name_data
    #     return redirect(url_for("index"))
    # return render_template("index1.html",form = form, name = session.get("name"))

if __name__=="__main__":
    app.run(debug=True)

运行后报错，根据错误提示，我查找了代码中的block 'title'，并没有被定义两次，然而还是报错，我就删除了代码中唯一的一处block 'title'，还是报错。