"Duplicate -wallet filename specified" -> "Wallet already loaded"

When you use `loadwallet` to try to load a wallet that has been loaded, a message saying _Duplicate -wallet filename specified_ shows up. In my opinion, this message is a little confusing, it should be changed to _Wallet has already been loaded_ or something like that.

Start at the loadwallet entry point and locate the path that reports “Duplicate -wallet filename specified” when the wallet is already loaded. Update the user-facing wording to clearly state that the wallet is already loaded, then verify that repeating loadwallet produces the new message.

Written by the indexing model from the issue text.