GetProvidersResult should return unique providers

[mikevoronov]

It seems that providers from GetProvidersResult could contain non-unique peers. I think that this behaviour could be error-prone for users of rust-libp2p (I've already stumbled with that:)) and providers should be unique.

[mxinden]

Given that we don't group the providers by response, we could replace the Vec with a HashSet within GetProviders.

@romanb what do you think?

[romanb]

Sounds ok to me. Furthermore, if we want to introduce a configurable or hard-coded limit on how many providers to look for, terminating the query early if enough providers are found (as I think other libp2p-kad implementations offer), we would need to de-duplicate anyway.

[mxinden]

#1528 would fix the duplication.

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Furthermore, if we want to introduce a configurable or hard-coded limit on how many providers to look for, terminating the query early if enough providers are found (as I think other libp2p-kad implementations offer), we would need to de-duplicate anyway.

👍