[abofh]

(Well) encrypted data is fundamentally indistinguishable from random data.

De-duplication requires commonality between files, which could not be found in encrypted data if users had unique keys.

Thus, if they have the ability to de-dupe _after_ you've uploaded a copy, they have the ability to decrypt your entire archive.

I'm not saying that's how they do it, but it would seem the logic is that your data never was particularly well encrypted.