Summary

UndirectedAdaptor<G> removes edge direction by chaining the incoming and outgoing traversals of the wrapped directed graph.

For self-loops, the same underlying edge is both incoming and outgoing, so the adaptor yields it twice.

This causes neighbors(...) and edges(...) to double-count a single self-loop edge.

Reproducer

use petgraph::{prelude::*, visit::{IntoEdges, IntoNeighbors, UndirectedAdaptor}};

fn main() {
    let mut g = DiGraph::<(), ()>::new();
    let a = g.add_node(());
    g.add_edge(a, a, ());

    let ug = UndirectedAdaptor(&g);

    let neighbors = ug.neighbors(a).collect::<Vec<_>>();
    let edges = ug.edges(a).collect::<Vec<_>>();

    println!("neighbors = {:?}", neighbors);
    println!("edges.len() = {}", edges.len());
}

Observed behavior:

• neighbors(a) contains a twice,
• edges(a) yields the same self-loop twice.

Possible root cause

The current implementation removes direction by concatenating:

• incoming neighbors / edges, and
• outgoing neighbors / edges,

but does not deduplicate the overlap case where both traversals refer to the same underlying self-loop.

That makes the adaptor overcount incident structure for valid inputs.

Suggested fix

Special-case self-loops when combining incoming and outgoing traversals so that the same underlying edge is yielded only once.