schema.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/schema.json b/schema.json
index 7a2b7c8..902a39d 100644
--- a/schema.json
+++ b/schema.json
@@ -251,7 +251,8 @@
       "type": "array",
       "items": {
         "$ref": "#/definitions/linkage"
-      }
+      },
+      "uniqueItems": true
     },
     "empty": {
       "description": "Describes an empty to-one relationship.",