Which transactions go into which blocks?

[bosshyip]

I read the paper and did a brief forum search, so I apologize if this has already been asked/answered.

How do nodes determine when to include a particular transaction into a block?

There scenario I'm wondering about is this:
Alice tries to double-spend her coins.  As luck would have it, two different nodes simultaneously hash the next block, 1001, each containing one transaction of Alice's double spending attempt.  Call them 1001a and 1001b.  Then, a node working on 1001a hashes the next block, 1002, and every node sees that the chain containing 1000 -> 1001a -> 1002 is the longest (and most likely to be honest) and halts work on 1001b.  One of Alice's two forks in her double-spending attempts is ignored, and her attempt therefore fails.  The guy who was running the node that made 1001b sees that he almost made 50 bc and gets annoyed, but I guess that's acceptable.

But what happens to the other transactions that were in block 1001b but not in 1001a?  Do they get lost as well?  Suppose Bob also bought something at about the same time as Alice, and his transaction ended up being hashed in 1001b (but not 1001a) as well.  I assume that what happens next is that after chain 1002 gets accepted as legitimate, a node working on 1003 overhears Alice's and Bob's transactions, and then checks to see if the two are already in the chain.  It will find that Alice's transaction is fake, and ignore it, and that Bob's transaction isn't in the chain yet, and add that to the block-in-progress.  But then that means that every transaction has to keep echoing around for at least a few blocks to make sure that it ends up in the accepted chain - and every node that overhears a transaction has to check to make sure it isn't already in the list.

The other question - and I know this must have been answered somewhere and I just couldn't find it - what determines transaction uniqueness?  Say Alice buys a widget from Bob for 31.42 bc.  Alice then decides she wants another widget, and places a new order - with the same keys for both buyer and seller - for another 31.42 bc.  What makes these two transactions distinct?  If a node sees one transaction for 31.42 from A to B, then sees a transaction for 31.42 from A to B, what tips it off that these are two distinct transactions rather than the same transaction detected twice?