init: ignore repeated `-addnode` startup values #36014

Startup -addnode values are copied directly into the connection manager's added-node list. Unlike runtime addnode add calls, this path does not reject repeated values.

For example:

-addnode=example.com
-addnode=example.com

Both entries are currently stored. While the destination is disconnected, ThreadOpenAddedConnections() processes each entry during every retry cycle.

This can cause redundant DNS lookups, connection attempts, and log messages. getaddednodeinfo also reports the repeated entry.

This change is complementary to, and independent of, #35600.

#35600 prevents overlapping manual connection attempts by tracking destinations while a connection attempt is in progress. This PR instead removes repeated -addnode values before the connection threads start, preventing duplicate stored entries and sequential redundant retries.

It does not replace or broaden #35600's in-flight connection handling.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36014.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35940 (net: allow selecting BIP152 high-bandwidth peers with -addnode by w0xlt)
• #31260 (scripted-diff: Type-safe settings retrieval by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Concept ACK

Comparing this patch to the validation performed in CConnman::AddNode() (the RPC path), which resolves the target — full resolution is out of scope as clearly explained in the description and commit body. What about also checking the port (perhaps using SplitHostPort), so e.g. node.example and node.example:8333 (mainnet default port) would be treated as the same target, and the second one excluded as a duplicate?