Is it seen as spammy, keyword stuffing, if the post/page title contains the same keywords as the website title? I have always setup WordPress to show the title of the post, followed by the title of the page. For example, using the keywords "toy cars" on a website named "Best Toy Cars", the title of a post would show as something like this: Top 10 Toy Cars for Christmas 2017 | Best Toy Cars Is that bad to have the keywords twice, even though they only appear once in the actual post title? Will the keyword density be too high in the overall title, and will that negatively affect the rankings? In other words: should the keywords only appear once in the complete title as seen on search engines?
No that doesn't factor at all. If the post title serves the post for good then its all good. It doesn't matter whether it matches your site title or not. And just for Keyword stuffing search engines like Google doesn't care about it any more unless you make it look like that it is just for ranking and gives a glimpse of automated post.