I have the duplication in the recently open list on my system too with 3.0.6. A testnet wallet is showing up twice. I think you should file a bug report for that. They should only show unique entries there.
Yes it appends this in the config file:
"recently_open": [
"C:\\users\\***\\downloads\\electrum_data\\wallets\\default_wallet",
"C:\\Users\\***\\Downloads\\electrum_data\\wallets\\Default_Wallet",
"C:\\Users\\***\\Downloads\\electrum_data\\wallets\\Bech32",
"C:\\Users\\***\\Downloads\\electrum_data\\wallets\\default_wallet"
],
default_wallet = Default_Wallet.
I think, as this is Windows, it doesn't cause an error, however, if in linux, there'd be an error if it is on linux (as in Windows "Default_Wallet" as a string in explorer.exe equals "default_wallet").
I'm not sure how this has happened though as it doesn't seem a reasonable thing to have happened.
EDIT: https://github.com/spesmilo/electrum/issues/4020