Return early on duplicate calls to funding_transaction_signed

We may produce duplicate `FundingTransactionReadyForSigning` events if
the user has processed an initial event but has not yet called back with
`funding_transaction_signed` and a peer reconnection occurs. If the user
also handles the duplicate events, any duplicate calls to
`funding_transaction_signed` after an initial successful one would
return an error. This doesn't make sense, as the API should remain
idempotent, so we return early on any duplicate calls.

Author: wpaulino

lightning/src/ln/channel.rs

@@ -8617,6 +8617,12 @@ where
 						.unwrap_or(false));
 				}
 
+				if signing_session.holder_tx_signatures().is_some() {
+					// Our `tx_signatures` either should've been the first time we processed them,
+					// or we're waiting for our counterparty to send theirs first.
+					return Ok((None, None));
+				}
+
 				signing_session
 			} else {
 				let err =