Problems

Bad Responsibilities Assignments
Code Duplication
Maintainability
Single Responsibility Violation.
Copy-pasted code.

Solutions

Refactor

Sample Code

Wrong

<?

final class SocialNetwork {

    function postStatus(string $newStatus) {
        if (!$user->isLogged()) {
            throw new Exception('User is not logged');
        }
        ///...
    }

    function uploadProfilePicture(Picture $newPicture) {
        if (!$user->isLogged()) {
            throw new Exception('User is not logged');
        }
        ///...
    }

    function sendMessage(User $recipient, Message $messageSend) {
        if (!$user->isLogged()) {
            throw new Exception('User is not logged');
        }
        ///...
    }
}

Right

<?

final class SocialNetwork {

    function postStatus(string $newStatus) {
        $this->assertUserIsLogged();
        ///...
    }

    function uploadProfilePicture(Picture $newPicture) {
        $this->assertUserIsLogged();
        ///...
    }

    function sendMessage(User $recipient, Message $messageSend) {
        $this->assertUserIsLogged();
        ///...
    }

    function assertUserIsLogged() {
        if (!$this->user->isLogged()) {
            throw new Exception('User is not logged');
            //This is just a simplification to show the code smell
            //Operations should be defined as objects with preconditions etc.
        }
    }
}

Detection

Some modern linters can detect repeated patterns (not just repeated code) and also while performing our code reviews we can easily detect this problem and ask for a refactor.

Conclusion

Adding a new feature should be straightforward it our model maps 1:1 to real world and our responsibilities are in the correct places.
We should be alert for small changes spanning in several classes.