Can a graph have multiple identical elements?
Graphs don't have values.  Graphs don't hold numbers.  You don't insert a value into a graph.  Graphs have vertices and edges.

You can augment a graph to associate each vertex (node) with a number, if you wish.  If you do that, it's up to you to define whether or not you allow two different vertices to have the same number or not.  There is no universal answer.