Is AES-XTS considered safe to encrypt multiple files with the same keys?
From a draft of the IEEE spec, 


  "An XTS-AES key shall not be associated with more than one key scope.
  The reason is that encrypting more than one block with the same key
  and the same index introduces security vulnerabilities that might
  potentially be used in an attack on the system. In particular, key
  reuse enables trivial cut-and-paste attacks."


You are seriously breaking the warranty here. I take it you know this...


  "The proof from Rogaway yields strong security guarantee as long as
  the same key is not used to encrypt much more than a terabyte of data
  (which gives q=236 blocks). For this case, no attack can succeed with
  probability better than 2^53 (i.e., approximately one in eight
  quadrillion).  This security guarantee deteriorates as more data is
  encrypted under the same key. For example, when using the same key for
  a petabyte of data, attacks such as in D.4.2 have success probability
  of at most approximately 2^37 (i.e., approximately eight in a
  trillion), and with exabyte, of data the success probability is at
  most approximately 2^17 (i.e., approximately eight in a million). The
  decision on the maximum amount of data to be encrypted with a single
  key should take into account the above calculations together with the
  practical implication of the described attack, (e.g. ability of the
  attacker to modify plaintext of a specific block, where the position
  of this block may not be under attacker’s control)."


So you may have some space before the tradeoff becomes completely broken I guess.

Also, if the cloud storage system can prevent anything from being written to the user's space without first encrypting it, (or prevent anything from being decrypted that it didn't itself encrypt), then it might be able to prevent some copy/paste attacks (from users). Since all encryption/decryption/writes are done remotely from the attacking user, it eliminates some of the IEEE worries about some types of passive attacks and random data attacks.

Ok, that's my most generous answer. You're asking a question of the form, "There's this security standard. Can I throw out a major piece of it and make it useful for something else?"

And while it's super tempting to launch into comparative cryptanalysis of AES-CBC and AES-XTS with your modifications, the only responsible answer to all such questions has to be, "No, that will give you something that's broken and unverified, equivalent to just making something up from scratch. You shouldn't want that. The first rule of crypto is don't build your own crypto."

I highly recommended approaching this from scratch as, "I want a mode or protocol that accomplishes X, Y, and Z, but I don't care about attacks A, B, or C. Is there any such mode? If not, what gets closest?" Maybe there isn't, and you have to face the hard choice of deduplication or security. But hey, who knows, you might find that there's something you hadn't even considered that meets your design objectives.

Best of luck!