@JohnPhilpin So you have to keep in mind that your existing Mastodon account (at Mastodon.social I think?) is a totally separate account. That account is @Fractals. You can cross post to it like you can to a twitter account (which I think is what you have set up now). That's a totally separate option from setting up ActivityPub support. (I will not that I follow your mastodon account and I haven't seen any double posting since you turned on cross-posting so either Mastodon.social is smart enough to ignore the extra posts or one of the two ways you're cross posting there is).

If you turn on ActivityPub support you will essentially be creating a NEW Mastodon account. It'll be @whatever@john.philpin.com (I don't beleive you can mess with the domain at the end, though I could be wrong about that). So you could be @fractals@john.philpin.com or you could be @johnphilpin. But whatever you choose it'll be a separate identity/account from your existing Mastodon account.

I beleive if you leave cross-posting on your posts will go to both accounts.