Test what happens when you have edges between same start and end nodes

> Is correct that this is a warning and not an error? By construction of the graph object, is it even possible to have two edges with the same u and v? I mean, can you have two edges that connect the same two nodes?

_Originally posted by @dalonsoa in https://github.com/ImperialCollegeLondon/SWMManywhere/pull/83#discussion_r1525790626_

Start with the discussion from PR #83 referenced in the issue, then locate the graph construction and the warning about edges sharing the same start and end nodes. Add coverage for the possible duplicate-edge case and document the expected warning or error behavior; done means the test resolves the open question.

Written by the indexing model from the issue text.