How handle errors when inserting data?

Error dump:

mysqli_sql_exception #1062
Duplicate entry 'john' for key 'users.username'

A part of my code:

Code:

$dataUser = [
'username' => $request->getPost('username'),
'email'    => $request->getPost('email'),
'password' => md5($request->getPost('password'))
];

$userModel = new UserModel();

$userModel->insert($dataUser);