Given the news recently about Paypal's upcoming deployment of capability to transfer BTC to external wallets, Apple's advertisement of an open  biz dev position related to alternative payments, Venmo's entrance into BTC, etc, I'm wondering what technology needs to be developed to facilitate easy transfer between these payment systems via BTC? Obviously the transactions would occur off-chain (Lightning), but the primary issue seems to be contact/identity management. Do DIDs fit the bill here assuming the payment network integrate those identities into their apps? Consider the use-case where two mobile device users want to transmit money from one to another across disparate payment networks. How do they add each other's contact/identity info?

Please forgive the newbie question: if address X has a balance of 10 BTC and the holder of that private key signs a transaction that sends 1 BTC from X to Y, what prevents that signed transaction from being rebroadcast later and another 1 BTC transferred to Y?

Thanks.