This is my first post and I wanted to do an automatic crosspost, as a beginner the hard truth is, current solutions are limited and paid, so I couldn't find an easy and fast solution so I'll list some things to consider.

Duplicate content gets punished

As stated by @MrConerMurphy first problem is that google penalizes duplicate content due to SEO (Search Engine Optimization), google shares the details to this issue here, so the short solution is the use of a Canonical URL on each site, if you use your own blog you may use the link of that blog, in my case I don't own a blog, and I'm currently writing in hashnode because is more user friendly, so I'll leave it as the referenced URL on Dev.to. To add a canonical URL on Dev.to you add this header to your text.

---
canonical_url: https://YOUR_URL
---

Formatting differences

At first glance hashnode has a more friendly format interface, Dev.to has some tips that are not so obvious at the first time using it and it also has an editor guide wich is more comprehensive, but for fast blogging using the simple hashnode editor then copy and paste the content to Dev.to seems enough.

Save your time and money

I was searching for a quick solution and came across CodeWrite and CrossPost, both good solutions but paid, my take is, the importance of a good post is getting to the point quick, because I came to Dev.to and Hashnode when I was searching for solutions for coding, personal blogs are fun, but the truth is when you search for a solution on the internet you always rather find it fast and keep on what you are doing.

That's it, I hope this saves you some time.