`UNION ALL` does not work when the same graph appears multiple times in the union

[pstutz]

We use the QGN or the graph itself as they key to store retaggings, so we cannot have different retaggings for the same graph which would be required when doing a UNION ALL of a graph with itself.

[pstutz]

How to reproduce:

val g = session.cypher("CONSTRUCT NEW () RETURN GRAPH").getGraph
val union = g.unionAll(g)
union.nodes("n").size shouldBe 2

Expected :2
Actual   :1