Sample Code:

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