if I buy it from itch.io do I need to buy it again from patreon and can I download the new version from patreon or just from itch.io
If you purchase the current version here on itch.io, you will get access to all future updates here (if there are ever going to be any).
As far as I understand Patreon's purchase mechanic, it will be the same over there.
From my own experience, if you're a Patreon supporter, you will get access to the game download on Patreon for as long as you are a paying supporter. After that you won't have access to the download.
The versions are identical on Patreon and itch.io.