The start value / IV for streaming modes like AES-CTR and AES-GCM must be a
nonce (number only used once). When the combination of encryption key and IV
is ever used twice, then the encryption can be reversed by simply applying
XOR. The image shows encrypted_image1 XOR encrypted_image2. The output of
encrypted_image1 XOR encrypted_image2 XOR original_image1 is
original_image2.
Some examples are double-plus bad because they also utilize PyCrypto. Friends don't let friends use PyCrypto. Please use https://github.com/pyca/cryptography