.../Value/RelationshipNormalizerValue.php          | 46 ++++++++++++++++++++--
 1 file changed, 43 insertions(+), 3 deletions(-)

diff --git a/src/Normalizer/Value/RelationshipNormalizerValue.php b/src/Normalizer/Value/RelationshipNormalizerValue.php
index 2549879..df0d653 100644
--- a/src/Normalizer/Value/RelationshipNormalizerValue.php
+++ b/src/Normalizer/Value/RelationshipNormalizerValue.php
@@ -74,9 +74,49 @@ class RelationshipNormalizerValue extends FieldNormalizerValue {
     // Empty 'to-many' relationships must be an empty array.
     // @link http://jsonapi.org/format/#document-resource-object-linkage
     $data = parent::rasterizeValue() ?: [];
-    return empty($data) && $this->cardinality == 1
-      ? ['data' => NULL, 'links' => $links]
-      : ['data' => $data, 'links' => $links];
+
+    if ($this->cardinality === 1) {
+      return empty($data)
+        ? ['data' => NULL, 'links' => $links]
+        : ['data' => $data, 'links' => $links];
+    }
+    else {
+      return ['data' => static::ensureUniqueResourceLinkage($data), 'links' => $links];
+    }
+  }
+
+  /**
+   * Ensures unique resource linkage.
+   *
+   * @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
+   */
+  protected static function ensureUniqueResourceLinkage(array $resource_identifier_objects) {
+    if (count($resource_identifier_objects) <= 1) {
+      return $resource_identifier_objects;
+    }
+
+    $unique_resource_identifier_objects = array_reduce($resource_identifier_objects, function ($carry, $resource_identifier_object) {
+      $id = $resource_identifier_object['id'];
+      if (!isset($carry[$id])) {
+        $carry[$id] = $resource_identifier_object;
+      }
+      else {
+        if (!isset($carry[$id]['meta'])) {
+          $carry[$id]['meta']['arity'] = 1;
+        }
+        $carry[$id]['meta']['arity']++;
+      }
+      return $carry;
+    }, []);
+
+    return array_values($unique_resource_identifier_objects);
   }
 
   /**