multi: fix `lnwallet.ErrDoubleSpend` #14791

Job	Run time
lint code	6m 32s
cross compilation	16m 11s
check pinned dependencies (google.golang.org/grpc v1.59.0)	5s
check commits	6m 58s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	9s
run macOS itest	39m 29s
check release notes updated	3s
RPC and mobile compilation check	6m 37s
sample configuration check	3m 18s
Sqlc check	4m 59s
run ubuntu itests (btcd, backend=btcd cover=1)	27m 28s
run unit tests (btcd unit-cover)	16m 21s
run ubuntu itests (bitcoind, backend=bitcoind cover=1)	31m 18s
run windows itest	34m 30s
run unit tests (unit tags="kvdb_etcd")	19m 24s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	31m 10s
run unit tests (unit tags="kvdb_postgres")	17m 50s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling" cover=1)	32m 35s
run unit tests (unit tags="kvdb_sqlite")	16m 43s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	42m 58s
run unit tests (btcd unit-race)	35m 18s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	58m 0s
run unit tests (unit-module)	3m 25s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	33m 48s
run ubuntu itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	55m 11s
run ubuntu itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	34m 23s
run ubuntu itests (neutrino, backend=neutrino cover=1)	31m 26s
finish	7s
	10h 6m 16s

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