Cryptonote which underlies Monero's protocol claims that it is double spend proof, so I want to disambiguate what this means as it's not clear to me from Cryptonote's overview page.

From what I understand a key image is generated to allow miners and other nodes to verify if a UTXO has already been spent. But does this refer only to transactions already on the blockchain, i.e. confirmed? Or does it mean that once a transaction is broadcast and is in the memory pool waiting to be mined to a block no other competing transactions will be accepted by the network - a form of instant traction if you will.

If it's the former how does the network deal with two closely broadcast transactions which reference the same UTXO(s)? Is this handled in a similar manner to Bitcoin?

Any clarification would be very helpful.