No, I plan on employing the same system that Tahoe does: I will optionally incorporate a random convergence secret. This effectively disables the deduplication

No, I plan on employing the same system that Tahoe does: I will optionally incorporate a random convergence secret. This effectively disables the deduplication properties, but provides a defense against these two attacks. This convergence secret can be added to the end of every capability token, or optionally omitted (in which case I use zeroes). So you have two options: allow deduplication but be susceptible to the confirmation of file attack/learn the remaining information attack, or more security but with duplication. Cryptographically this feeds in as a salt/initialization vector to HKDF along with the entire plaintext. HKDF is then used to generate a key and iv for use with AES