Parcels bound for private gateways aren't deduped

[gnarea]

We should dedupe them like we dedupe CCAs.

[gnarea]

This is actually desirable.

Imagine a Letro user changes their profile picture, and then immediately changes it again. If the original parcel is in the store (i.e., it hasn't been collected by the private gateway), we want to replace it with the new parcel. However, if the private gateway has already collected the original parcel, we still want to send the new one so that Letro can get the latest version of the profile picture.