Reduce delegate invocations and enumeration.

[paulomorgado]

The existing code is using 2 delegate invocations when, as it is, only one is needed.

var duplicates = endpoints
                .GroupBy(e => GetEndpointName(e))
                .Where(g => g.Key != null && g.Count() > 1);

The grouping of null endpoints could be avoided, but the overall cost could be highier:

var duplicates = endpoints
    .Select(e => (endpoint: e, key: GetEndpointName(e)))
    .Where(i => i.key is not null)
    .GroupBy(i => i.Key, i => i.endpoint)
    .Where(g => g.Count() > 1);