Why is it terrible for SEO to have the same pages on my two sites?

Question

Search Engine Optimization

On one of our sites there is a catalog of seminars (each in the form of a separate landing page inside the main site), they have been in the search for a long time, indexed. We also created a new website (personal blog), we also want to place the same catalog of seminars on it, for the convenience of users. How to do it better?
1) Make full copies of landing pages on the new site
2) Leave the catalog on the old site, and redirect on the new one
3) Make a catalog on the new site, and redirect from the old one
Thank you.

Answer 1

Why is it terrible for SEO to have the same pages on my two sites?

Sooner or later the pages will stick together, i.e. only one site will be found in the search for queries related to these pages.
Not redirects, but links.
On the new one, write only a list, and for a detailed description, let them go to landing pages.