Error in test class while creating user. Error says duplicate username but no duplicate exists

Question

@istest private class Penalizacion_Memo_isTest {

@istest static void test_Memo(){
    user u = new user();
    u.MEMO__c=TRUE;
    u.LastName = 'Test Code';
    u.Email = '[email protected]';
    u.Alias = 'Tcode';
    u.Username = '[email protected]';
    u.CommunityNickname = 'test12';
    u.LocaleSidKey = 'en_US';
    u.TimeZoneSidKey = 'GMT';
    u.ProfileID = '00e36000000ao2F';
    u.LanguageLocaleKey = 'en_US';
    u.EmailEncodingKey = 'UTF-8';
    insert u;
}
}

This is my code. I've already changed the username a couple of times and still receive the same error even though I'm absolutely positive there is no user with that username in any of our organizations.

Error:

System.DmlException: Insert failed. First exception on row 0; first error: DUPLICATE_USERNAME, Duplicate Username.
The username already exists in this or another Salesforce organization. Usernames must be unique across all Salesforce organizations. To resolve, use a different username (it doesn't need to match the user's email address). : [Username] Stack Trace: Class.Penalizacion_Memo_isTest.test_Memo: line 16, column 1

Answer 1

User name must be unique across all Salesforce organizations which Salesforce has created till now

Append Datetime.Now() with username String to make it unique

Answer 2

User name is unique across all orgs. Random usernames that has test domains and 12345 numerics might be an issue sometimes. I faced the similar issue in the past. Its better to build emails and usernames in such a way that they are unique and chance of possible duplication is low. I prefer appending with company name as domain or adding system.now() or current time at the end. If its a user name, you can add sandbox name as well.

Instead of [email protected], try something like

u.Username = testuser@mycompnay+string.valueof(system.now());