If we consider information to be safe if we encrypt it (e.g., text in a file, encrypted with modern strong encryption), would it be safer (as in harder to crack) if we then encrypted the encrypted file, and encrypted the encrypted^2 file, etc.? Is this what strong encryption already does behind the scenes?
This scheme would only really improve security when using multiple different keys, as well as obviously two different algorithms. Doing the same thing over and over only grows linearly, while encryption is based on the fact that the attacker needs exponentially (not mathematically speaking) more effort to crack than was put in by the encrypting party. So if the attacker can crack it once he can also do it again with no further effort. Furthermore, most of the time the problem with encryption is not the actual cipher, but rather the key storage and distribution. Keepass for example only uses a single encryption layer (AES or ChaCha) for the database, instead offering a very robust portfolio for key derivation (basically making a big key from a small password, or translating entropy to something usable for the cipher, while keeping the determinism)
But that is essentially what two-factor authentication does. And you can also use this with fully symmetrical encryption to an extent (look up how OTP works with keepass for an example)
There is also some pretty good literature from the early days of the us military opsec, where they lay out very well the incremental steps into doing it better and better.
Hope that helps, but I am not qualified to be cited on this information :)