No idea at all. My blog does really well on Google, so I think if either site is being penalized for duplicate content it's probably the Substack.

It looks like 8% of my newsletter signups are "from the app" according to the Substack dashboard - which I think is how they show signups that they've encouraged as opposed to signups I had myself.

I'm really just using Substack because they've solved email deliverability and they're free for me to use to send out emails.

If you have a blog with RSS and just need something that auto-emails people your updates you can run my project on a raspberry pi. No need to tie yourself into the substack ecosystem if you don't want to

[0] https://github.com/MattSayar/rsspberry2email