方法createUserWithEmailAndPassword返回的错误具有code属性。根据文档，错误codeauth/email-already-in-use：
如果已存在具有给定电子邮件地址的帐户，则抛出。
您至少可以使用if/else或switch等条件语句来检查code，并向用户显示/log/dispatch/etc消息或代码：

fire.auth().createUserWithEmailAndPassword(this.state.email, this.state.password)
  .then(u => {})
  .catch(error => {
     switch (error.code) {
        case 'auth/email-already-in-use':
          console.log(`Email address ${this.state.email} already in use.`);
          break;
        case 'auth/invalid-email':
          console.log(`Email address ${this.state.email} is invalid.`);
          break;
        case 'auth/operation-not-allowed':
          console.log(`Error during sign up.`);
          break;
        case 'auth/weak-password':
          console.log('Password is not strong enough. Add additional characters including special characters and numbers.');
          break;
        default:
          console.log(error.message);
          break;
      }
  });

希望能有所帮助！

对于firebase admin sdk，有一个更简单的答案：

const uidExists = auth().getUser(uid).then(() => true).catch(() => false))

const emailExists = auth().getUserByEmail(email).then(() => true).catch(() => false))

我是这样使用fetchSignInMethodsForEmail的：

import { getAuth, fetchSignInMethodsForEmail } from 'firebase/auth';

const auth = getAuth();
let signInMethods = await fetchSignInMethodsForEmail(auth, email);
if (signInMethods.length > 0) {
  //user exists
} else {
   //user does not exist
}

参考文件

from firebase_admin import auth

user = auth.get_user_by_email(email)
print('Successfully fetched user data exists: {0}'.format(user.uid))

在python管理服务器中