watchtower/wtclient: dedup backups across restarts

Now that the committed and acked updates are persisted across restarts,
we will use them to filter out duplicate commit heights presented by the
client.

watchtower/wtclient/client_test.go

@@ -1246,6 +1246,55 @@ var clientTests = []clientTest{
 			h.assertUpdatesForPolicy(hints, h.clientCfg.Policy)
 		},
 	},
+	{
+		// Asserts that the client will deduplicate backups presented by
+		// a channel both in memory and after a restart. The client
+		// should only accept backups with a commit height greater than
+		// any processed already processed for a given policy.
+		name: "dedup backups",
+		cfg: harnessCfg{
+			localBalance:  localBalance,
+			remoteBalance: remoteBalance,
+			policy: wtpolicy.Policy{
+				BlobType:     blob.TypeDefault,
+				MaxUpdates:   5,
+				SweepFeeRate: 1,
+			},
+		},
+		fn: func(h *testHarness) {
+			const (
+				numUpdates = 10
+				chanID     = 0
+			)
+
+			// Generate the retributions that will be backed up.
+			hints := h.advanceChannelN(chanID, numUpdates)
+
+			// Queue the first half of the retributions twice, the
+			// second batch should be entirely deduped by the
+			// client's in-memory tracking.
+			h.backupStates(chanID, 0, numUpdates/2, nil)
+			h.backupStates(chanID, 0, numUpdates/2, nil)
+
+			// Wait for the first half of the updates to be
+			// populated in the server's database.
+			h.waitServerUpdates(hints[:len(hints)/2], 5*time.Second)
+
+			// Restart the client, so we can ensure the deduping is
+			// maintained across restarts.
+			h.client.Stop()
+			h.startClient()
+			defer h.client.ForceQuit()
+
+			// Try to back up the full range of retributions. Only
+			// the second half should actually be sent.
+			h.backupStates(chanID, 0, numUpdates, nil)
+
+			// Wait for all of the updates to be populated in the
+			// server's database.
+			h.waitServerUpdates(hints, 5*time.Second)
+		},
+	},
 }
 
 // TestClient executes the client test suite, asserting the ability to backup