Ignoring (but warn) on duplicate -wallet parameters

Summary:
I expect that there are many users with load on startup wallet definitions in bitcoin.conf or via startup CLI argument.
With the new settings.json r/w configuration file, users unloading and loading a wallet through the GUI or via the RPC calls might end up with a duplicate -wallet entry (one that still remains in bitcoin.conf or CLI) plus the new duplication in settings.json due to the unload/load.

Steps to reproduce

• create wallet (if via RPC set load_on_startup or unloadwallet/loadwallet then set load_on_startup).
• stop bitcoin
• start bitcoind again with same --wallet=mywallet

I guess it is acceptable to skip duplicates.

This is a backport of core#20199

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10718