Duplicate Username error in one sandbox but not in the other

Question

I am trying to validate a change set and it's giving me a Duplicate Username error. The problem is that the same test passes in 2 other sandboxes. From my understanding if the same username is being used for the test method in a different sandbox instance the error should be present there as well. However this is not the case. Here's the error.

System.DmlException: Insert failed. First exception on row 0; first error: 
DUPLICATE_USERNAME, Duplicate Username.<br>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] 

Any ideas why this is happening?

EDIT I changed the username and pushed a new change set. The test passed. The sandbox I am pushing to is FULL and the sandbox I am pushing from is a Partial Copy sandbox.

Answer 1

If this only errors in the 1 box it's most likely that the user actually exists out side of the test class in that specific org. Test this by printing the user to the debug log with system.debug and then querying to see if it exists.

Regardless of the reason, the issue is self explanatory. Here is some sample logic I've used in the past.

public static User newUser() {
    Id profileId = [
        SELECT Id
        FROM Profile
        WHERE Name = 'System Administrator'
    ].Id;

    String uniqueEmail = String.valueOf(Math.round(Math.random()*1000)) +
        string.valueOf(System.now().format('ddMMYYYhhmmss')) +
        '@' + URL.getSalesforceBaseUrl().getHost();

    User result = new User(
        FirstName = 'Grumple',
        LastName = 'Frillskin',
        Alias = 'gfrilly',
        Email = uniqueEmail,
        UserName = uniqueEmail,
        EmailEncodingKey='UTF-8',
        LanguageLocaleKey='en_US',
        LocaleSidKey='en_US',
        TimezoneSidKey='America/Los_Angeles',
        ProfileId = profileId
    );

    return result;
}

Notes:

• Avoid generic terms such as "Test" or "Sample"
• Use the org url so that the chance of the user existing outside this org is minimal
• Use random number + time to make the username as unique as possible
• FirstName, LastName, Email, and Alias do not need to be unique\
• Using the host url does run the risk of duplicates in the event of a refresh (as you will not be on the same host and someone else on your new host may be using similar logic) consider adding the org ID as well.