.../Value/RelationshipNormalizerValue.php          | 59 +++++++++++++---------
 1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/src/Normalizer/Value/RelationshipNormalizerValue.php b/src/Normalizer/Value/RelationshipNormalizerValue.php
index 2cad6e9..e260de3 100644
--- a/src/Normalizer/Value/RelationshipNormalizerValue.php
+++ b/src/Normalizer/Value/RelationshipNormalizerValue.php
@@ -102,36 +102,47 @@ class RelationshipNormalizerValue extends FieldNormalizerValue {
       return $resource_identifier_objects;
     }
 
-    $unique_resource_identifier_objects = array_reduce($resource_identifier_objects, function ($carry, $resource_identifier_object) {
+    $resource_identifier_arity = static::computeResourceLinkageArity($resource_identifier_objects);
+
+    $assigned_arities = [];
+    foreach ($resource_identifier_objects as $index => $resource_identifier_object) {
       $id = $resource_identifier_object['id'];
-      $has_meta = isset($resource_identifier_object['meta']);
 
-      if (!isset($carry[$id])) {
-        $carry[$id] = $resource_identifier_object;
-        if ($has_meta) {
-          // All resource identifier objects' metas need to be nested.
-          unset($carry[$id]['meta']);
-        }
+      // If arity === 1, don't set [meta][arity].
+      if ($resource_identifier_arity[$id] === 1) {
+        continue;
       }
-      else {
-        // Increment arity.
-        if (!isset($carry[$id]['meta']['arity'])) {
-          $carry[$id]['meta']['arity'] = 1;
-        }
-        $carry[$id]['meta']['arity']++;
-
-        // Nest all resource identifier objects' metas under a shared key.
-        if ($has_meta) {
-          if (!isset($carry[$id]['meta']['metas'])) {
-            $carry[$id]['meta']['metas'] = [];
-          }
-          $carry[$id]['meta']['metas'][] = $resource_identifier_object['meta'];
-        }
+
+      // Determine arity of current resource identifier and assign it.
+      $arity = isset($assigned_arities[$id])
+        ? ++$assigned_arities[$id]
+        : 0;
+      $resource_identifier_objects[$index]['meta']['arity'] = $arity;
+
+      // Track assigned arities.
+      $assigned_arities[$id] = $arity;
+    }
+
+    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;
       }
+      $carry[$id]++;
       return $carry;
     }, []);
-
-    return array_values($unique_resource_identifier_objects);
   }
 
   /**