Index uniqueness not working as expected.

[dxtr-1-0]

I have registered a property key and in the same management transaction made it indexed (composite) and unique.

val propertySchemaEntry = management.makePropertyKey("id_property")
                .dataType(String.class)
                .cardinality(Cardinality.SINGLE)
                .make();
management.buildIndex("id_property_index",  Vertex.class)
                    .addKey(propertySchemaEntry)
                    .unique()
                    .buildCompositeIndex();

Now I am trying to add vertices with the same id_property property with the expectation that this won't happen. But janusgraph is creating as many vertices with different ids as I am trying. On retrieving by the same property, I am getting only the latest vertex created but the older vertices still exist (verified using actual vertex ID).

val createdId1 = g.addV("user")
            .property(ID_PROPERTY, "U123")
            .property(CREATED_PROPERTY, System.currentTimeMillis())
            .property(UPDATED_PROPERTY, System.currentTimeMillis()).next().id();
g.tx().commit();
g.tx().close();
Thread.sleep(500);
val createdId2 = g.addV("user")
            .property(ID_PROPERTY, "U123")
            .property(CREATED_PROPERTY, System.currentTimeMillis())
            .property(UPDATED_PROPERTY, System.currentTimeMillis()).next().id();
g.tx().commit();
g.tx().close();
val id1 = (Long) g.V().has(ID_PROPERTY, "U123").next().id();
val id2 = (Long) g.V().has(ID_PROPERTY, "U123").next().id();
Assertions.assertEquals(id1, id2);
Assertions.assertEquals(createdId1, createdId2);

id1 and id2 are the same which is the latest vertex id i.e. createdId2. But createdId1 and createdId2 are different and there is no warning/error. How is the uniqueness being assured here?
Edit: I even tried setting the consistency modifier to LOCK for this index but that also does not help. Although I feel that if unique is explicitly mentioned at the time of creation, the consistency modifier should be automatically set to LOCK.

Database -> Hbase Mini cluster
Janusgraph version -> 0.6.1
Am I missing something or is this expected? My expectation was that the second creation would either overwrite/add the properties to the same vertex with a warning log. Or worst case throw an Error.