diff --git a/nitro/wallet/wallet.nim b/nitro/wallet/wallet.nim
index ca84f1c..6707e4a 100644
--- a/nitro/wallet/wallet.nim
+++ b/nitro/wallet/wallet.nim
@@ -36,11 +36,12 @@ func sign(wallet: Wallet, state: SignedState): SignedState =
   signed.signatures &= wallet.key.sign(state.state)
   signed
 
-func createChannel(wallet: var Wallet, state: SignedState): ChannelId =
-  let signed = wallet.sign(state)
-  let id = getChannelId(signed.state.channel)
-  wallet.channels[id] = signed
-  id
+func createChannel(wallet: var Wallet, state: SignedState): ?!ChannelId =
+  let id = getChannelId(state.state.channel)
+  if wallet.channels.contains(id):
+    return ChannelId.failure("channel with id " & $id & " already exists")
+  wallet.channels[id] = wallet.sign(state)
+  ok id
 
 func updateChannel(wallet: var Wallet, state: SignedState) =
   let signed = wallet.sign(state)
@@ -52,7 +53,7 @@ func openLedgerChannel*(wallet: var Wallet,
                         chainId: UInt256,
                         nonce: UInt48,
                         asset: EthAddress,
-                        amount: UInt256): ChannelId =
+                        amount: UInt256): ?!ChannelId =
   let state = startLedger(wallet.address, hub, chainId, nonce, asset, amount)
   wallet.createChannel(state)
 
@@ -63,7 +64,7 @@ func acceptChannel*(wallet: var Wallet, signed: SignedState): ?!ChannelId =
   if not verifySignatures(signed):
     return ChannelId.failure "incorrect signatures"
 
-  wallet.createChannel(signed).success
+  wallet.createChannel(signed)
 
 func latestSignedState*(wallet: Wallet, channel: ChannelId): ?SignedState =
   wallet.channels?[channel]
diff --git a/tests/nitro/testWallet.nim b/tests/nitro/testWallet.nim
index 64852fd..84965ef 100644
--- a/tests/nitro/testWallet.nim
+++ b/tests/nitro/testWallet.nim
@@ -21,7 +21,7 @@ suite "wallet: opening ledger channel":
 
   setup:
     wallet = Wallet.init(key)
-    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, amount)
+    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, amount).get
 
   test "sets correct channel definition":
     let definition = wallet.state(channel).get.channel
@@ -42,6 +42,9 @@ suite "wallet: opening ledger channel":
     check wallet.state(channel).get.appDefinition == EthAddress.zero
     check wallet.state(channel).get.appData.len == 0
 
+  test "does not allow opening a channel that already exists":
+    check wallet.openLedgerChannel(hub, chainId, nonce, asset, amount).isErr
+
 suite "wallet: accepting incoming channel":
 
   let key = PrivateKey.random()
@@ -71,6 +74,10 @@ suite "wallet: accepting incoming channel":
     signed.signatures = @[key.sign(State.example)]
     check wallet.acceptChannel(signed).isErr
 
+  test "fails when channel with this id already exists":
+    check wallet.acceptChannel(signed).isOk
+    check wallet.acceptChannel(signed).isErr
+
 suite "wallet: making payments":
 
   let key = PrivateKey.random()
@@ -85,7 +92,7 @@ suite "wallet: making payments":
   test "paying updates the channel state":
     wallet = Wallet.init(key)
     let me = wallet.address
-    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 100.u256)
+    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 100.u256).get
 
     check wallet.pay(channel, asset, hub, 1.u256).isOk
     check wallet.balance(channel, asset, me) == 99.u256
@@ -97,14 +104,14 @@ suite "wallet: making payments":
 
   test "paying updates signatures":
     wallet = Wallet.init(key)
-    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 100.u256)
+    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 100.u256).get
     check wallet.pay(channel, asset, hub, 1.u256).isOk
     let expectedSignature = key.sign(wallet.state(channel).get)
     check wallet.signature(channel, wallet.address) == expectedSignature.some
 
   test "pay returns the updated signed state":
     wallet = Wallet.init(key)
-    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 42.u256)
+    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 42.u256).get
     let updated = wallet.pay(channel, asset, hub, 1.u256).option
     check updated?.state == wallet.state(channel)
     check updated?.signatures == wallet.signatures(channel)
@@ -130,7 +137,7 @@ suite "wallet: making payments":
 
   test "payment fails when payer has insufficient funds":
     wallet = Wallet.init(key)
-    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 1.u256)
+    channel = wallet.openLedgerChannel(hub, chainId, nonce, asset, 1.u256).get
     check wallet.pay(channel, asset, hub, 1.u256).isOk
     check wallet.pay(channel, asset, hub, 1.u256).isErr
 
@@ -148,7 +155,7 @@ suite "wallet: accepting payments":
     payer = Wallet.init(payerKey)
     receiver = Wallet.init(receiverKey)
     channel = payer.openLedgerChannel(
-      receiver.address, chainId, nonce, asset, 100.u256)
+      receiver.address, chainId, nonce, asset, 100.u256).get
     let update = payer.latestSignedState(channel).get
     discard receiver.acceptChannel(update)
 
@@ -184,12 +191,12 @@ suite "wallet: accepting payments":
 
   test "does not accept a payment for an unknown channel":
     let newChannel = payer.openLedgerChannel(
-      receiver.address, chainId, nonce + 1, asset, 100.u256)
+      receiver.address, chainId, nonce + 1, asset, 100.u256).get
     let payment = payer.pay(newChannel, asset, receiver.address, 10.u256).get
     check receiver.acceptPayment(newChannel, asset, payer.address, payment).isErr
 
   test "does not accept a payment for a different channel":
     let newChannel = payer.openLedgerChannel(
-      receiver.address, chainId, nonce + 1, asset, 100.u256)
+      receiver.address, chainId, nonce + 1, asset, 100.u256).get
     let payment = payer.pay(newChannel, asset, receiver.address, 10.u256).get
     check receiver.acceptPayment(channel, asset, payer.address, payment).isErr