Sorry for interrupting. Personally, I don’t think Kickstarter is being unfair to those who purchased the game on itch.io. Normally, to get early access, Kickstarter backers have to pledge at a tier higher than the base game price, so this might understandably cause some frustration among early-access tier backers and itch.io buyers.
Of course, if you plan to run a Kickstarter, it would be good to explain to your backers in advance that those who supported the game early on itch.io helped sustain the project up to this point. You could also take steps like removing sales on itch.io. This way, you can protect the interests of both itch.io supporters and Kickstarter backers.
And if the campaign doesn’t succeed, you can always try again later or continue selling early access as you are now. At the very least, you’ve made an effort to secure funding for the project.
That’s just my suggestion.