通过使用电子邮件地址作为关键字，您可以确保每个电子邮件地址只有一个条目:

By using the email address as a key, you can make sure there will be only one entry for each email address:

var fbRef = new Firebase('https://someurl.firebaseio.com/');

// create a new user somehow
var newUser = {
    fullName: "John Doe", 
    company: "Pet's Place", 
    email: "john@petsplace.org"
}

// escape '.' because it can not be used as a key in FB
function escapeEmail(email){
    return email.replace(/\./g, ',');
}

fbRef.child('accounts').child(escapeEmail(newUser.email)).set(newUser, function(err) {
    if(err) {
        // if there was an error, the email address already exists
    }
);

然后在您的规则中验证密钥(电子邮件地址)不存在:

Then in your rules validate that the key (email address) does not already exist:

{
    "rules": {
        ".read": true,
        "accounts": {
            "$account": {
                ".write": true,
                ".validate": "!root.child('accounts').child(newData.child('email').val().replace(/\./g, ',')).exists()"
            }
        }
    }
}

另请查看此处以获取更多信息:什么 Firebase 规则将防止基于其他字段的集合中的重复项?和这里:如何防止 Firebase 中出现重复的用户属性?

Also have a look here for some additional information: What Firebase rule will prevent duplicates in a collection based on other fields? and here: How do you prevent duplicate user properties in Firebase?