Add warning for conflicting entries

[martinheidegger]

Conflicting entries are currently deterministically resolved using the "highest relevant" node:

https://github.com/martinheidegger/hyper-dns/blob/main/resolve-context.js#L84-L88

however, as this may be unexpected behavior it may be better to add a warning to the output (like CORS configuration) and even with debugging disabled it may be a good idea throw an error message in that case.