If the plaintext contains the key used to encrypt the ciphertext, is the ciphertext easier to decrypt? I have a symmetric key which can be used to encrypt plaintext and decrypt the resulting ciphertext.
I use it to encrypt a plaintext which is a copy of the aforementioned key.
My adversary gets a copy of the resulting ciphertext.


Assuming my adversary knows that the plaintext is a copy of the key,
is there a way they can guess my key more easily than if the plaintext wasn't the key?

Note: I am new to cryptography so if there is a flaw in my question I will be happy to clarify.
There aren't any known generic attacks against this setup to work on arbitrary ciphers. On the other hand, the standard notion of IND-CPA security does not imply security when encrypting your own key.

I don't know of any commonly used block or stream cipher which has trouble with this setup, but one could be created and might even exist.