openchannel: reject duplicate outpoints in open channel request

Duplicate outpoints in the request lead to fee miscalculation and an
invalid PSBT with the same input listed twice. Validate early and return
a clear error message.

staticaddr/openchannel/manager.go

@@ -299,6 +299,18 @@ func (m *Manager) OpenChannel(ctx context.Context,
 				err)
 		}
 
+		// Check for duplicate outpoints which would lead to fee
+		// miscalculation and an invalid PSBT with the same input
+		// listed twice.
+		seen := make(map[wire.OutPoint]struct{}, len(outpoints))
+		for _, op := range outpoints {
+			if _, ok := seen[op]; ok {
+				return nil, fmt.Errorf("duplicate outpoint "+
+					"%v in request", op)
+			}
+			seen[op] = struct{}{}
+		}
+
 		deposits, allActive =
 			m.cfg.DepositManager.AllOutpointsActiveDeposits(
 				outpoints, deposit.Deposited,

staticaddr/openchannel/manager_test.go

@@ -210,6 +210,34 @@ func testOutPoint(b byte) wire.OutPoint {
 	}
 }
 
+func TestOpenChannelDuplicateOutpoints(t *testing.T) {
+	t.Parallel()
+
+	op := testOutPoint(1)
+	manager := &Manager{
+		cfg: &Config{},
+	}
+
+	req := &lnrpc.OpenChannelRequest{
+		NodePubkey:         make([]byte, 33),
+		LocalFundingAmount: 100000,
+		SatPerVbyte:        10,
+		Outpoints: []*lnrpc.OutPoint{
+			{
+				TxidStr:     op.Hash.String(),
+				OutputIndex: op.Index,
+			},
+			{
+				TxidStr:     op.Hash.String(),
+				OutputIndex: op.Index,
+			},
+		},
+	}
+
+	_, err := manager.OpenChannel(context.Background(), req)
+	require.ErrorContains(t, err, "duplicate outpoint")
+}
+
 func TestValidateInitialPsbtFlags(t *testing.T) {
 	t.Parallel()