Avoid redundant broadcast of local commitment transaction

This change follows the rationale of commit 62236c7 and addresses the
last remaining redundant local commitment broadcast.

There's no need to broadcast our local commitment transaction if we've
already seen a confirmed one as it'll be immediately rejected as a
duplicate/conflict.

This will also help prevent dispatching spurious events for bumping
commitment and HTLC transactions through anchor outputs since the
dispatch for said events follows the same flow as our usual commitment
broadcast.

Author: wpaulino

lightning/src/chain/channelmonitor.rs

@@ -2325,6 +2325,17 @@ impl<Signer: Sign> ChannelMonitorImpl<Signer> {
 					log_trace!(logger, "Updating ChannelMonitor: channel force closed, should broadcast: {}", should_broadcast);
 					self.lockdown_from_offchain = true;
 					if *should_broadcast {
+						// There's no need to broadcast our commitment transaction if we've seen one
+						// confirmed (even with 1 confirmation) as it'll be rejected as
+						// duplicate/conflicting.
+						let detected_funding_spend = self.funding_spend_confirmed.is_some() ||
+							self.onchain_events_awaiting_threshold_conf.iter().find(|event| match event.event {
+								OnchainEvent::FundingSpendConfirmation { .. } => true,
+								_ => false,
+							}).is_some();
+						if detected_funding_spend {
+							continue;
+						}
 						self.broadcast_latest_holder_commitment_txn(broadcaster, logger);
 						// If the channel supports anchor outputs, we'll need to emit an external
 						// event to be consumed such that a child transaction is broadcast with a