Prevent duplicate publish.credentials and repositories.credentials from being saved to the config database

[Gedochao]

Fixes #4252

When a duplicate is passed to publish.credentials and repositories.credentials keys, a warning is printed and the duplicate is not actually saved.

[warn]: an identical publish.credentials entry for host a is already configured, a duplicate won't be saved.
Skipped saving config for key publish.credentials

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)

How much have your relied on LLM-based tools in this contribution?

extensively, Cursor + Claude

How was the solution tested?

Added new automated tests and ran them