walletrpc+btcwallet: no custom account with various key scopes

Currently, a user can create a custom account with various key scopes.
This is not a desired behaviour.

lnwallet/btcwallet/btcwallet.go

@@ -843,6 +843,10 @@ func (b *BtcWallet) ListAddresses(name string,
 // The address type can usually be inferred from the key's version, but may be
 // required for certain keys to map them into the proper scope.
 //
+// For custom accounts, we will first check if there is no account with the same
+// name (even with a different key scope). No custom account should have various
+// key scopes as it will result in non-deterministic behaviour.
+//
 // For BIP-0044 keys, an address type must be specified as we intend to not
 // support importing BIP-0044 keys into the wallet using the legacy
 // pay-to-pubkey-hash (P2PKH) scheme. A nested witness address type will force
@@ -860,6 +864,22 @@ func (b *BtcWallet) ImportAccount(name string, accountPubKey *hdkeychain.Extende
 	dryRun bool) (*waddrmgr.AccountProperties, []btcutil.Address,
 	[]btcutil.Address, error) {
 
+	// For custom accounts, we first check if there is no existing account
+	// with the same name.
+	if name != lnwallet.DefaultAccountName &&
+		name != waddrmgr.ImportedAddrAccountName {
+
+		_, err := b.ListAccounts(name, nil)
+		if err == nil {
+			return nil, nil, nil,
+				fmt.Errorf("account '%s' already exists",
+					name)
+		}
+		if !waddrmgr.IsError(err, waddrmgr.ErrAccountNotFound) {
+			return nil, nil, nil, err
+		}
+	}
+
 	if !dryRun {
 		accountProps, err := b.wallet.ImportAccount(
 			name, accountPubKey, masterKeyFingerprint, addrType,