Encryption with the same key and repeated nonce/counter produce the same cipher stream. Ciphertext in GCM (or CTR) mode is cipherstream XOR plaintext, thus give

Encryption with the same key and repeated nonce/counter produce the same cipher stream. Ciphertext in GCM (or CTR) mode is cipherstream XOR plaintext, thus given two ciphertexts with the same key/nonce: ciphertext1 XOR ciphertext2 = (cipherstream XOR plaintext1) XOR (cipherstream XOR plaintext2) = plaintext1 XOR plaintext2 In GCM it can also break authentication.