For your purposes, yes, identical hashes means identical files.
As other answers make clear, it's possible to construct 2 different files which result in the same hash and MD5 is not particularly robust in this regard.
So use a stronger hashing algorithm if you plan on comparing a large number of excel documents or if you think someone might want to manipulate the comparison. SHA1 is better than MD5. SHA256 is better again and should give you complete confidence for your particular usage.
