A "graph" can sometimes implicitly mean a "Simple Graph" which has neither loops nor multiple edges between pairs of vertices, but in some contexts can mean "Mu

A "graph" can sometimes implicitly mean a "Simple Graph" which has neither loops nor multiple edges between pairs of vertices, but in some contexts can mean "Multi-Graph", which does allow multiple edges and loops. So the usual conversion of the bridges problem to a network gives us a "Multi-Graph" ... the distinction is often left to be inferred, because it's usually obvious (to the skilled practitioner). One way around this to end up with a simple graph is to include a vertex in the middle of each bridge. Then each edge has one end in the landmass and the other end in the middle of the bridge. That equivalence/conversion shows that the distinction, in this case, is effectively unimportant.