diff --git a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
index ea8cbbb360..211de1f58b 100644
--- a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
+++ b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
@@ -172,7 +172,7 @@ public function post(Request $request, $entity_type_id, $bundle, $field_name) {
     // Create the file.
     $file_uri = "{$destination}/{$filename}";
 
-    $temp_file_path = $this->streamUploadData($file_uri);
+    $temp_file_path = $this->streamUploadData();
 
     // This will take care of altering $file_uri if a file already exists.
     file_unmanaged_prepare($temp_file_path, $file_uri);
@@ -189,14 +189,14 @@ public function post(Request $request, $entity_type_id, $bundle, $field_name) {
     ];
     $file = File::create($values);
 
-    // Validate the file entity against entiy level validation and field level
+    // Validate the file entity against entity level validation and field level
     // validators.
     $this->validate($file, $field_definition);
 
-    // Move the file to the correct location after validation.
-    // Move the file to the correct location based on the file entity,
-    // replacing any existing file.
-    if (!file_unmanaged_move($temp_file_path, $file_uri)) {
+    // Move the file to the correct location after validation. Use
+    // FILE_EXISTS_ERROR as the file location has already been determined above
+    // in file_unmanaged_prepare().
+    if (!file_unmanaged_move($temp_file_path, $file_uri, FILE_EXISTS_ERROR)) {
       throw new HttpException(500, 'Temporary file could not be moved to file location');
     }
 
diff --git a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
index 1ec01d3eda..59019e3fe2 100644
--- a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
@@ -174,6 +174,43 @@ public function testPostFileUpload() {
     $this->assertSame($this->testFileData, file_get_contents('public://foobar/example.txt'));
   }
 
+  /**
+   * Tests using the file upload POST route with a duplicate file.
+   *
+   * A new file should be created with a suffixed name.
+   */
+  public function testPostFileUploadDuplicateFile() {
+    $this->initAuthentication();
+
+    $this->provisionResource([static::$format], static::$auth ? [static::$auth] : [], ['POST']);
+
+    $this->setUpAuthorization('POST');
+
+    $uri = Url::fromUri('base:' . static::$postUri);
+
+    // This request will have the default 'application/octet-stream' content
+    // type header.
+    $response = $this->fileRequest($uri, $this->testFileData);
+
+    $this->assertSame(201, $response->getStatusCode());
+
+    // Make the same request again. The file should be saved as a new file
+    // entity that has the same file name but a suffixed file URI.
+    $response = $this->fileRequest($uri, $this->testFileData);
+
+    $this->assertSame(201, $response->getStatusCode());
+
+    // Loading expected normalized data for file 2.
+    $expected = $this->getExpectedNormalizedEntity(2);
+    // Update the file URI to match the generated name.
+    $expected['uri'][0]['value'] = 'public://foobar/example_0.txt';
+
+    $this->assertResponseData($expected, $response);
+
+    // Check the actual file data.
+    $this->assertSame($this->testFileData, file_get_contents('public://foobar/example_0.txt'));
+  }
+
   /**
    * Tests using the file upload route with a zero byte file.
    */
@@ -266,15 +303,21 @@ protected function getExpectedUnauthorizedAccessMessage($method) {
 
   /**
    * Gets the expected file entity.
+   *
+   * @param int $fid
+   *   The file ID to load and create normalized data for.
+   *
+   * @return array
+   *   The expected normalized data array.
    */
-  protected function getExpectedNormalizedEntity() {
+  protected function getExpectedNormalizedEntity($fid = 1) {
     $author = User::load(static::$auth ? $this->account->id() : 0);
-    $file = File::load(1);
+    $file = File::load($fid);
 
     $expected_normalization = [
       'fid' => [
         [
-          'value' => 1,
+          'value' => (int) $file->id(),
         ],
       ],
       'uuid' => [