Problems with using AES Key as IV in CBC-Mode
CBC mode encrypts as follows:
$$
C_0 = E_K(IV\oplus P_0);\\
C_i = E_K(C_{i-1}\oplus P_i),
$$
where $P_i$ are plaintext blocks and $C_i$ are ciphertext blocks. Traditionally, IV must be random and is published alongside the ciphertext to enable decryption. If it is also published in your case, then this reveals the key and is trivially insecure.

If the $IV=K$ is not attached to the ciphertext, then you get a deterministic (in contrast to probabilistic) encryption  scheme. It has a number of insecure properties, such as


the ciphertext does not change if the same plaintext is encrypted multiple times.
if two plaintexts have the same prefix, then the ciphertexts will have the same property.


Maybe there are others, but these ones are sufficient to avoid using such scheme to protect confidentiality of any reasonable amount of data, except maybe for very short (16 bytes or less) plaintexts.