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The transaction that gets included in the block earlier by the witness producing the block.

Really interesting question. My guess is that asset transfers would be limited to one per block. Maybe there are a sub-type of transactions that are easier to bundle. E.g. if you and I play a game that ends in the transfer of an asset, then maybe all of our moves within the game can be bundled into a single block, while the asset transfer operation is included in its own block.

Any chance STEEMIT will make it so that only 1 of each unique coustom_json id per block?

In the end it depends on the software hosted by the witness to order the transactions. Whichever it deems first will be the one to be executed. In terms of custom json token transfers, both will be included in the block but the API of the server will only accept the first tx in the block.

Any chance STEEMIT will make it so that only 1 of each unique coustom_json id per block?