—————signup.htmlを編集----------

使うなら、その後base.htmlの作成。
継承する内容を記載する。

user登録・重複登録防止処理を記載する。

----------Base.html—————

<!-- Bootstrap CSS -->
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3" crossorigin="anonymous">

<title>Hello, world!</title>

{%block header%}
{% endblock header%}

{% block content %}
{% endblock content %}

—————signup.html—————

{% extends 'base.html' %}

{%block content%}

{{error}}

<main class="form-signin">
  <form method='POST'>{% csrf_token %}
    <h1 class="h3 mb-3 fw-normal">Please sign in</h1>
    <div class="form-floating">
      <input type="text" class="form-control" id="floatingInput" name='username' placeholder="name@example.com">
      <label for="floatingInput">username</label>
    </div>
    <div class="form-floating">
      <input type="password" class="form-control" id="floatingPassword" name='password' placeholder="Password">
      <label for="floatingPassword">Password</label>
    </div>
    <button class="w-100 btn btn-lg btn-primary" type="submit">Sign in</button>
    <p class="mt-5 mb-3 text-muted">&copy; 2017–2021</p>
  </form>
</main>

{%endblock content%}

----------Views.py----------

from sqlite3 import IntegrityError
from django.shortcuts import render
from django.contrib.auth.models import User
from django.db import IntegrityError

Create your views here.

def signupfanc(request):
object_list = User.objects.all()
print(object_list)

if request.method == 'POST':
    username = request.POST['username']
    password = request.POST['password']
    
    try:
        user = User.objects.create_user(username,'',password)
    except IntegrityError:
        return render(request,'signup.html',{'error':'このユーザーはすでに登録済みです。'})
else:
    print('not POST')
    
return render(request,'signup.html',{})