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As the topic says, after a long time in the game the Il Duca gun and the classic mobster outfit aren't available anymore.
They just disappeard from the arms dealer/the closet.
Restarting the game didn't solve the problem and I'm out of ideas.

Can anyone help me?
Installed any mods?
In Galaxy, try the 'Manage installation' > 'Verify/Repair' function.
Or back up save files and reinstall the game using the offline installers.