The more robust cryptography, therefore, is designed to prevent exactly what you seek to accomplish.
This is usually not done because determining that an encrypted document is identical to a known plaintext one effectively allows "decrypting" the document, or being sure of its content, defeating the cryptography.
It is sometimes possible to add a (either signed or unprotected) "hash signature" that represents the unencrypted content of the file. The more secure encryption schemes explicitly use a "non-deterministic" (actually they endeavour to be "truly random", not just use a crypto-level PRNG, by collecting entropy through various means) approach, and will never encrypt the same data in the same way twice. "I have been working on testing whether same encrypted files are same without decrypting it"
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