No need to put twice.
diff --git a/jena-arq/src/main/java/org/apache/jena/riot/out/JsonLDWriter.java b/jena-arq/src/main/java/org/apache/jena/riot/out/JsonLDWriter.java
index 68ebc0f..b5d56cc 100644
--- a/jena-arq/src/main/java/org/apache/jena/riot/out/JsonLDWriter.java
+++ b/jena-arq/src/main/java/org/apache/jena/riot/out/JsonLDWriter.java
@@ -155,7 +155,6 @@
                         // typed literal)
                         Map<String, Object> x2 = new LinkedHashMap<>() ;
                         x2.put("@id", p.getURI()) ;
-                        x2.put("@id", p.getURI()) ;
                         if (! isLangString(o) && ! isSimpleString(o) ) 
                             // RDF 1.1 : Skip if rdf:langString or xsd:string.
                             x2.put("@type", literalDatatypeURI) ;