Username uniqueness across Salesforce
Key Takeaway
A username must be unique across every Salesforce org, including trial and sandbox orgs. The same email address can be on users in many orgs.
A username is the login name for a Salesforce user, and it is written like an email address. It must be unique across every Salesforce org in existence, not just within your own org. Because of this, the same person often needs a different username in each org they use.
Username rules
A username must be unique across all Salesforce orgs, including trial and sandbox orgs. If a username is already taken in any other org, you can't use it in yours.
A username must also be in the format of an email address, for example jane@salesforce.com. It doesn't have to be a working mailbox.