Remove duplicate targets from endpoints for headless services

[thomasv314]

Fixes #1015

PR #645 introduced an issue where endpoints for headless services could return duplicate target IP addresses, which the Route53 API does not allow. This PR ensures that an endpoint can only point to a unique set of target IP addresses.

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[thomasv314]

I signed it

[thomasv314]

/assign @njuettner

[njuettner]

one small nit, PTAL

[njuettner]

@thomasv314 thanks for your PR 👍
/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: njuettner, thomasv314

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