Would hashing a password twice prevent naive attacks? A common way of using MD5 or any other hashing algorithms to store passwords is to hash the password with a salt $$\text{hash}(\text{password} \mathbin\| \text{salt}).$$ This way, if an attacker wants to brute force the passwords, they have to do this for each password since each password has a different salt. With this setup, a determined attacker could still brute force a password since they have the salt and they know the hashed password was generated with $\text{hash}(\text{password} \mathbin\| \text{salt}).$
Would storing the hashed password as $$\text{hash}(\text{hash}(\text{password} \mathbin\| \text{salt}))$$ be a way of deterring the attacker? The idea with this approach is that the attacker doesn't know the password is being hashed twice. This means that if they try to brute force the password with the assumption that the hashed password is of the form $\text{hash}(\text{password} \mathbin\| \text{salt})$, they will never be able to crack it.
We don't approach our security through obscurity, we live in a world that Kerckhoffs's Principles rules! We assume that everything is known by the attackers except the key/password is secret. Your modification only hardens the search a little; just one more hash!
Password hashing has gone a longs way and in modern password hashing, we want them

memory-hard so that massive GPU/ASIC searches are almost prohibited.
Non-Parallelizable so that the attacker cannot parallel even a single search due to the requirement of multiple threads for a single password hashing.
Many Iterations so that the attacker must need huge time for the calculation of a single password hash.

All of the above parameters can be adjustable ( depend on the password hashing algorithm) so that the system designers can fit them into their system, too. For example, having a huge iteration that takes longer than 1 second is not good from the user's perspective. 250K or 1M may be good depending on the server.
Don't fall into Schneier's Law on Security

Any person can invent a security system so clever that she or he can't think of how to break it.

Instead of designing your own, first understand the standard ones like Scrypt, PBKDF2, and the winner of the 2015's Password Hashing Competition; Argon2.