An encryption relies on taking the input and transforming it in sequence to produce an encrypted response.

If you take the encrypted result and run it through the same thing that just encrypted it, you don't get a stronger encryption because you haven't changed the possible number of outputs.

Example: if I encrypt the following sentence by increasing each letter +1 modulo (A to B, Z to A)

WE LOVE CAT MEMES

becomes

XF MPWF DBU NFNFT

This is a pretty simple encryption and is easy to reverse.

If I use the same process to re-encrypt this output, I get:

YG NQXG ECV OGOGU

This hasn't increased in complexity because it is evident that the algorithm I used only changed from "Add +1 Modulo along the sequence of A-Z" to "Add +2 Modulo along the sequence of A-Z". It's the same basic function with just a constant (the +1 or +2) altered slightly. The output has the same possible number of results.

That's for using the same thing to re-encrypt.

Using two or more different forms of encryption should improve the results assuming one of them is 'stronger' to begin with. But I'm not confident on how exactly that would function. I assume their strength would either be combined or the stronger one would set the level of strength.