Database.DMLOptions dml = new Database.DMLOptions();
dml.DuplicateRuleHeader.allowSave = true;
dml.DuplicateRuleHeader.runAsCurrentUser = true;
Account duplicateAccount = new Account(Name='dupe');
Database.SaveResult sr = Database.insert(duplicateAccount, dml);
if (sr.isSuccess()) {
  System.debug('Duplicate account has been inserted in Salesforce!');
}