This article explores the Account/Organisation Duplicate Rules which can be configured and activated in your Salesforce environment.
Typically Account/Organisation Duplicate Rules will identify Accounts/Organisations that are being saved with the same name as an existing Account/Organisation, but this can be extended to include other duplicate detection logic, like phone number, street address, etc.

When an Account/Organisation record is saved the duplicate rule will run and provide an alert if a possible duplicate is detected. You then have the option of ignoring the alert and saving the new record or accessing the existing record.