User uploading a new representation overrides the previous one

This collections: https://forum.decentraland.org/t/collection-ubiq-cyberpunk-armor-created-by-intern-is-ready-for-review/1844

It looks like when the user adds a representation, it overrides the other body shape one.

Also they started having a problem with the file size of the item.. i'm worried that this change (https://github.com/decentraland/builder/commit/2542373cce034e951291f6764acc08430b2970a0) introduced an issue where now blobs are duplicated (since they are repeated in the `item.contents` mappings) and that's throwing an error regarding the size of the item. When calculating the size of the item we should exclude repeated blobs (we could create a `Map` to get rid of repeated blobs).

The files have been shared via slack with the team

Start by tracing the representation-upload flow and the item size calculation, focusing on the item.contents mappings and commit 2542373cce034e951291f6764acc08430b2970a0. Confirm whether adding one representation replaces another and whether repeated blobs inflate the reported size; done means both representations remain available and duplicate blobs are excluded from the size calculation.

Written by the indexing model from the issue text.