Firefly III can automatically stop duplicate transactions from being created.

app/Api/V1/Controllers/TransactionController.php

@@ -28,6 +28,7 @@ use FireflyIII\Api\V1\Requests\TransactionStoreRequest;
 use FireflyIII\Api\V1\Requests\TransactionUpdateRequest;
 use FireflyIII\Events\StoredTransactionGroup;
 use FireflyIII\Events\UpdatedTransactionGroup;
+use FireflyIII\Exceptions\DuplicateTransactionException;
 use FireflyIII\Helpers\Collector\GroupCollectorInterface;
 use FireflyIII\Models\TransactionGroup;
 use FireflyIII\Models\TransactionJournal;
@@ -279,7 +280,20 @@ class TransactionController extends Controller
         Log::channel('audit')
            ->info('Store new transaction over API.', $data);
 
-        $transactionGroup = $this->groupRepository->store($data);
+        try {
+            $transactionGroup = $this->groupRepository->store($data);
+        } catch (DuplicateTransactionException $e) {
+            // return bad validation message.
+            // TODO use Laravel's internal validation thing to do this.
+            $response = [
+                'message' => 'The given data was invalid.',
+                'errors'  => [
+                    'transactions.0.description' => [$e->getMessage()],
+                ],
+            ];
+
+            return response()->json($response, 422);
+        }
 
         event(new StoredTransactionGroup($transactionGroup));
 

app/Api/V1/Requests/TransactionStoreRequest.php

@@ -58,8 +58,9 @@ class TransactionStoreRequest extends Request
     public function getAll(): array
     {
         $data = [
-            'group_title'  => $this->string('group_title'),
-            'transactions' => $this->getTransactionData(),
+            'group_title'             => $this->string('group_title'),
+            'error_if_duplicate_hash' => $this->boolean('error_if_duplicate_hash'),
+            'transactions'            => $this->getTransactionData(),
         ];
 
         return $data;
@@ -75,6 +76,7 @@ class TransactionStoreRequest extends Request
         $rules = [
             // basic fields for group:
             'group_title'                          => 'between:1,1000|nullable',
+            'error_if_duplicate_hash'              => [new IsBoolean],
 
             // transaction rules (in array for splits):
             'transactions.*.type'                  => 'required|in:withdrawal,deposit,transfer,opening-balance,reconciliation',

app/Exceptions/DuplicateTransactionException.php

@@ -0,0 +1,31 @@
+<?php
+/**
+ * DuplicateTransactionException.php
+ * Copyright (c) 2019 thegrumpydictator@gmail.com
+ *
+ * This file is part of Firefly III (https://github.com/firefly-iii).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+namespace FireflyIII\Exceptions;
+use Exception;
+
+/**
+ * Class DuplicateTransactionException
+ */
+class DuplicateTransactionException extends Exception
+{
+
+}

app/Factory/TransactionGroupFactory.php

@@ -23,6 +23,7 @@ declare(strict_types=1);
 
 namespace FireflyIII\Factory;
 
+use FireflyIII\Exceptions\DuplicateTransactionException;
 use FireflyIII\Models\TransactionGroup;
 use FireflyIII\User;
 
@@ -52,10 +53,13 @@ class TransactionGroupFactory
      * @param array $data
      *
      * @return TransactionGroup
+     * @throws DuplicateTransactionException
      */
     public function create(array $data): TransactionGroup
     {
         $this->journalFactory->setUser($this->user);
+        $this->journalFactory->setErrorOnHash($data['error_if_duplicate_hash']);
+
         $collection = $this->journalFactory->create($data);
         $title      = $data['group_title'] ?? null;
         $title      = '' === $title ? null : $title;

app/Factory/TransactionJournalFactory.php

@@ -26,10 +26,12 @@ namespace FireflyIII\Factory;
 
 use Carbon\Carbon;
 use Exception;
+use FireflyIII\Exceptions\DuplicateTransactionException;
 use FireflyIII\Exceptions\FireflyException;
 use FireflyIII\Models\Account;
 use FireflyIII\Models\TransactionCurrency;
 use FireflyIII\Models\TransactionJournal;
+use FireflyIII\Models\TransactionJournalMeta;
 use FireflyIII\Models\TransactionType;
 use FireflyIII\Repositories\Account\AccountRepositoryInterface;
 use FireflyIII\Repositories\Bill\BillRepositoryInterface;
@@ -72,6 +74,8 @@ class TransactionJournalFactory
     private $typeRepository;
     /** @var User The user */
     private $user;
+    /** @var bool */
+    private $errorOnHash;
 
     /**
      * Constructor.
@@ -81,7 +85,8 @@ class TransactionJournalFactory
      */
     public function __construct()
     {
-        $this->fields = [
+        $this->errorOnHash = false;
+        $this->fields      = [
             // sepa
             'sepa_cc', 'sepa_ct_op', 'sepa_ct_id',
             'sepa_db', 'sepa_country', 'sepa_ep',
@@ -119,6 +124,7 @@ class TransactionJournalFactory
      * @param array $data
      *
      * @return Collection
+     * @throws DuplicateTransactionException
      */
     public function create(array $data): Collection
     {
@@ -193,11 +199,15 @@ class TransactionJournalFactory
      * @param NullArrayObject $row
      *
      * @return TransactionJournal|null
+     * @throws Exception
+     * @throws DuplicateTransactionException
      */
     private function createJournal(NullArrayObject $row): ?TransactionJournal
     {
         $row['import_hash_v2'] = $this->hashArray($row);
 
+        $this->errorIfDuplicate($row['import_hash_v2']);
+
         /** Some basic fields */
         $type            = $this->typeRepository->findTransactionType(null, $row['type']);
         $carbon          = $row['date'] ?? new Carbon;
@@ -376,6 +386,30 @@ class TransactionJournalFactory
         return $journal;
     }
 
+    /**
+     * If this transaction already exists, throw an error.
+     *
+     * @param string $hash
+     *
+     * @throws DuplicateTransactionException
+     */
+    private function errorIfDuplicate(string $hash): void
+    {
+        if (false === $this->errorOnHash) {
+            return;
+        }
+        $result = null;
+        if ($this->errorOnHash) {
+            /** @var TransactionJournalMeta $result */
+            $result = TransactionJournalMeta::where('data', json_encode($hash, JSON_THROW_ON_ERROR))
+                                            ->with(['transactionJournal', 'transactionJournal.transactionGroup'])
+                                            ->first();
+        }
+        if (null !== $result) {
+            throw new DuplicateTransactionException(sprintf('Duplicate of transaction #%d.', $result->transactionJournal->transaction_group_id));
+        }
+    }
+
     /**
      * @param TransactionCurrency|null $currency
      * @param Account                  $account
@@ -485,4 +519,14 @@ class TransactionJournalFactory
             throw new FireflyException(sprintf('Destination: %s', $this->accountValidator->destError)); // @codeCoverageIgnore
         }
     }
+
+    /**
+     * @param bool $errorOnHash
+     */
+    public function setErrorOnHash(bool $errorOnHash): void
+    {
+        $this->errorOnHash = $errorOnHash;
+    }
+
+
 }

app/Repositories/TransactionGroup/TransactionGroupRepository.php

@@ -27,6 +27,7 @@ namespace FireflyIII\Repositories\TransactionGroup;
 use Carbon\Carbon;
 use DB;
 use Exception;
+use FireflyIII\Exceptions\DuplicateTransactionException;
 use FireflyIII\Exceptions\FireflyException;
 use FireflyIII\Factory\TransactionGroupFactory;
 use FireflyIII\Models\AccountMeta;
@@ -314,6 +315,7 @@ class TransactionGroupRepository implements TransactionGroupRepositoryInterface
      * @param array $data
      *
      * @return TransactionGroup
+     * @throws DuplicateTransactionException
      */
     public function store(array $data): TransactionGroup
     {

app/Repositories/TransactionGroup/TransactionGroupRepositoryInterface.php

@@ -23,6 +23,7 @@ declare(strict_types=1);
 
 namespace FireflyIII\Repositories\TransactionGroup;
 
+use FireflyIII\Exceptions\DuplicateTransactionException;
 use FireflyIII\Models\TransactionGroup;
 use FireflyIII\Support\NullArrayObject;
 use FireflyIII\User;
@@ -125,6 +126,7 @@ interface TransactionGroupRepositoryInterface
      * @param array $data
      *
      * @return TransactionGroup
+     * @throws DuplicateTransactionException
      */
     public function store(array $data): TransactionGroup;