What is an example of elgamal homomorphic double encryption?
You are looking for commutative encryption rather than homomorphic encryption. Also Elgamal is probabilistic, which means in each encryption a fresh randomness is added to the ciphertext, which means if the same plaintext is encrypted twice, the ciphertexts will be different. 

For a deterministic commutative encryption scheme, you can use RSA encryption (SRA in your comment).