.../Value/RelationshipNormalizerValue.php          | 70 ++++++++++------------
 .../Value/RelationshipNormalizerValueTest.php      | 12 ++--
 2 files changed, 38 insertions(+), 44 deletions(-)

diff --git a/src/Normalizer/Value/RelationshipNormalizerValue.php b/src/Normalizer/Value/RelationshipNormalizerValue.php
index e260de3..3f44258 100644
--- a/src/Normalizer/Value/RelationshipNormalizerValue.php
+++ b/src/Normalizer/Value/RelationshipNormalizerValue.php
@@ -81,68 +81,62 @@ class RelationshipNormalizerValue extends FieldNormalizerValue {
         : ['data' => $data, 'links' => $links];
     }
     else {
-      return ['data' => static::ensureUniqueResourceLinkage($data), 'links' => $links];
+      return ['data' => static::ensureUniqueResourceIdentifierObjects($data), 'links' => $links];
     }
   }
 
   /**
-   * Ensures unique resource linkage.
+   * Ensures each resource identifier object is unique.
+   *
+   * The official JSON API JSON-Schema document requires that no two resource
+   * identifier objects are duplicated.
+   *
+   * This adds an @code arity @endcode member to each object's
+   * @code meta @endcode member. The value of this member is an integer that is
+   * incremented by 1 (starting from 0) for each repeated resource identifier
+   * sharing a common @code type @endcode and @code id @endcode.
    *
    * @param array $resource_identifier_objects
    *   A list of JSON API resource identifier objects.
+   *
    * @return array
    *   A set of JSON API resource identifier objects, with those having multiple
    *   occurrences getting [meta][arity].
    *
    * @see http://jsonapi.org/format/#document-resource-object-relationships
    * @see https://github.com/json-api/json-api/pull/1156#issuecomment-325377995
+   * @see https://www.drupal.org/project/jsonapi/issues/2864680
    */
-  protected static function ensureUniqueResourceLinkage(array $resource_identifier_objects) {
+  protected static function ensureUniqueResourceIdentifierObjects(array $resource_identifier_objects) {
     if (count($resource_identifier_objects) <= 1) {
       return $resource_identifier_objects;
     }
 
-    $resource_identifier_arity = static::computeResourceLinkageArity($resource_identifier_objects);
+    // Count each repeated resource identifier and track their array indices.
+    $analysis = [];
+    foreach ($resource_identifier_objects as $index => $rio) {
+      $composite_key = $rio['type'] . ':' . $rio['id'];
 
-    $assigned_arities = [];
-    foreach ($resource_identifier_objects as $index => $resource_identifier_object) {
-      $id = $resource_identifier_object['id'];
-
-      // If arity === 1, don't set [meta][arity].
-      if ($resource_identifier_arity[$id] === 1) {
-        continue;
-      }
-
-      // Determine arity of current resource identifier and assign it.
-      $arity = isset($assigned_arities[$id])
-        ? ++$assigned_arities[$id]
+      $analysis[$composite_key]['count'] = isset($analysis[$composite_key])
+        ? $analysis[$composite_key]['count'] + 1
         : 0;
-      $resource_identifier_objects[$index]['meta']['arity'] = $arity;
 
-      // Track assigned arities.
-      $assigned_arities[$id] = $arity;
+      // The index will later be used to assign an arity to repeated resource
+      // identifier objects. Doing this in two phases prevents adding an arity
+      // to objects which only occur once.
+      $analysis[$composite_key]['indices'][] = $index;
     }
 
-    return $resource_identifier_objects;
-  }
-
-  /**
-   * Computes the arity of each resource link.
-   *
-   * @param array $resource_identifier_objects
-   *   A list of JSON API resource identifier objects.
-   * @return array
-   *   Resource identifiers as keys, arities as values.
-   */
-  protected static function computeResourceLinkageArity(array $resource_identifier_objects) {
-    return array_reduce($resource_identifier_objects, function ($carry, $resource_identifier_object) {
-      $id = $resource_identifier_object['id'];
-      if (!isset($carry[$id])) {
-        $carry[$id] = 0;
+    // Assign an arity to objects whose type + ID pair occurred more than once.
+    foreach ($analysis as $computed) {
+      if ($computed['count'] > 0) {
+        foreach ($computed['indices'] as $arity => $index) {
+          $resource_identifier_objects[$index]['meta']['arity'] = $arity;
+        }
       }
-      $carry[$id]++;
-      return $carry;
-    }, []);
+    }
+
+    return $resource_identifier_objects;
   }
 
   /**
diff --git a/tests/src/Unit/Normalizer/Value/RelationshipNormalizerValueTest.php b/tests/src/Unit/Normalizer/Value/RelationshipNormalizerValueTest.php
index eb4f87c..855abfd 100644
--- a/tests/src/Unit/Normalizer/Value/RelationshipNormalizerValueTest.php
+++ b/tests/src/Unit/Normalizer/Value/RelationshipNormalizerValueTest.php
@@ -91,13 +91,13 @@ class RelationshipNormalizerValueTest extends UnitTestCase {
     $img1->rasterizeValue()->willReturn(['type' => 'file--file', 'id' => $img_id, 'meta' => ['alt' => 'Cute llama', 'title' => 'My spirit animal']]);
     $img1->getInclude()->willReturn(NULL);
     $img1->getCacheContexts()->willReturn(['ccimg1']);
-    $img1->getCacheTags()->willReturn(['ccimg1']);
+    $img1->getCacheTags()->willReturn(['ctimg1']);
     $img1->getCacheMaxAge()->willReturn(100);
     $img2 = $this->prophesize(RelationshipItemNormalizerValue::class);
     $img2->rasterizeValue()->willReturn(['type' => 'file--file', 'id' => $img_id, 'meta' => ['alt' => 'Adorable llama', 'title' => 'My spirit animal 😍']]);
     $img2->getInclude()->willReturn(NULL);
     $img2->getCacheContexts()->willReturn(['ccimg2']);
-    $img2->getCacheTags()->willReturn(['ccimg2']);
+    $img2->getCacheTags()->willReturn(['ctimg2']);
     $img2->getCacheMaxAge()->willReturn(50);
 
     $links = [
@@ -160,7 +160,7 @@ class RelationshipNormalizerValueTest extends UnitTestCase {
       ],
         (new CacheableMetadata())
           ->setCacheContexts(['ccimg1', 'user'])
-          ->setCacheTags(['ccimg1', 'relationship:foo'])
+          ->setCacheTags(['ctimg1', 'relationship:foo'])
           ->setCacheMaxAge(100),
       ],
       'multiple cardinality, all same values, with meta' => [
@@ -173,7 +173,7 @@ class RelationshipNormalizerValueTest extends UnitTestCase {
         ],
         (new CacheableMetadata())
           ->setCacheContexts(['ccimg1', 'user'])
-          ->setCacheTags(['ccimg1', 'relationship:foo'])
+          ->setCacheTags(['ctimg1', 'relationship:foo'])
           ->setCacheMaxAge(100),
       ],
       'multiple cardinality, some same values with same values but different meta' => [
@@ -187,7 +187,7 @@ class RelationshipNormalizerValueTest extends UnitTestCase {
         ],
         (new CacheableMetadata())
           ->setCacheContexts(['ccimg1', 'ccimg2', 'user'])
-          ->setCacheTags(['ccimg1', 'ccimg2', 'relationship:foo'])
+          ->setCacheTags(['ctimg1', 'ctimg2', 'relationship:foo'])
           ->setCacheMaxAge(50),
       ],
       'all the edge cases!' => [
@@ -204,7 +204,7 @@ class RelationshipNormalizerValueTest extends UnitTestCase {
         ],
         (new CacheableMetadata())
           ->setCacheContexts(['ccbar', 'ccfoo', 'ccimg1', 'ccimg2', 'user'])
-          ->setCacheTags(['ccimg1', 'ccimg2', 'ctbar', 'ctfoo', 'relationship:foo'])
+          ->setCacheTags(['ctbar', 'ctfoo', 'ctimg1', 'ctimg2', 'relationship:foo'])
           ->setCacheMaxAge(10),
       ],
     ];