De-duplication requires verifying that two files are identical. If they can verify that two files are identical they either compared them both in unencrypted form or they can compare the two encrypted files. The first method would require them to be able to unencrypt files without prompting all users for their private key to compare the copies.
The second method would require identical files from different users to encrypt to the same form, meaning the encryption is not key dependent and thus can also be decrypted without your key.
There may be some other method I'm missing but I don't see how they could de-duplicate with key-dependent encryption when their users hold their own keys.
The second method would require identical files from different users to encrypt to the same form, meaning the encryption is not key dependent and thus can also be decrypted without your key.
There may be some other method I'm missing but I don't see how they could de-duplicate with key-dependent encryption when their users hold their own keys.