My problem:

The site is organized in sections. Sections are managed with WordPress categories.

I have three different lists, mnatching the sections of the website.

Whenever I publish a new post in a section, a post notification email is sent to subscribers of the matching list, with a single post of that category.

My problem is that users can subscribe to more than one list. And posts may be posted in more than one category.

So subscribers may receive more than one post notification referring to the same post.

Possible solution 1:

Define in a variable the category of the "recent posts". This way I would have one single post notification email for all my categories.

Possible solution 2:

Have a check to verify before sending if the same contents of the post notification email are sent to the same person an avoid those particular cases (duplicates).