The nodes are SD blocks.
The validity of the transactions is already verified in the Safecoin layer. Here, we’re only concerned about the timeline.
You wouldn’t need to stop people from adding a transaction again: first, it would cost them money (the cost of adding an SD block), and then when somebody looked up the transaction, both the old and the new one would come up as a result. So yes, you could add it twice, but there would be no point.
For this to work, we’ll need to be able to search for contents in some way; it is currently missing, but @dirvine commented on something I wrote earlier today, and he said that stuff like that will be possible.