SQL-GR calls the initializeVertex() method once per cogroup (partition), and passes to it a single VertexState. A VertexState can contain only one Vertex, and therefore the result of the initializeVertex() call can be only one vertex (or 0 vertices) in the graph. This is true even if the cogroup contained multiple rows from the vertices table and therefore the iterator that contained vertex information also contained multiple rows.
You are responsible for figuring out how to express the information from multiple rows in a single vertex. In some cases this might involve discarding information; in other cases it may involve concatenating or adding information.
When initializing vertices, to avoid errors, do not change the vertex key of a vertex.