For a given Key K and two messages $m_1$, $m_2$ is it possible for an encryption algorithm (say AES256) to produce this: $AES(K, m_1) = AES(K, m_2)$.
Not if $m_1$ is different than $m_2$. Assuming you are talking about AES as a block cipher, and $m_1$ and $m_2$ as input blocks, this is not possible. Every different input block will have a unique output block. Take this cropped image as an example of a 4-bit block cipher with a fixed key, with 16 plaintexts and 16 ciphertexts:
As you can see, 11 encrypts into 12, and thus 12 decrypts to 11. If, as per your example, 13 also encrypted to 12, how would you correctly decrypt 12? Also, how would you decrypt 9?
Well, you couldn't, because a block cipher is a bijection, that is the amount of plaintexts and ciphertexts are composed of sets of equal size, and there is a one-to-one mapping between them, with all of them being paired.
The following behaviors of a block cipher are possible:
$AES(K_1, m_1) = AES(K_2, m_1)$
$AES(K_1, m_1) = AES(K_2, m_2)$
$AES(K_1, m_1) = m_1$
As fgrieu pointed out, a mode of operation with a third input (such as an IV or a prior block) may cause this to happen within a group of blocks, but since the decryption process depends on that third input as well, it will decrypt properly.