MockNetworkProvider should not throw on seen transaction resubmission

[mainnet-pat]

Referring to these lines: https://github.com/CashScript/cashscript/blob/7aeea22e1d14443b466642977369c888a08c37a6/packages/cashscript/src/network/MockNetworkProvider.ts#L48-L50

Full nodes do not throw if different clients or the same client submits the transaction already seen by it, because of the nature of p2p transaction propagation. Throwing is not appropriate here, a console warning would suffice.