An existing attribute with the same name is overwritten.
See also
Vertex, edge and graph attributes
delete_edge_attr(),
delete_graph_attr(),
delete_vertex_attr(),
edge_attr(),
edge_attr<-(),
edge_attr_names(),
graph_attr(),
graph_attr<-(),
graph_attr_names(),
igraph-attribute-combination,
igraph-dollar,
igraph-vs-attributes,
set_edge_attr(),
set_vertex_attr(),
set_vertex_attrs(),
vertex_attr(),
vertex_attr<-(),
vertex_attr_names()

