If I have multiple blog posts that target same keyword is it really bad for SEO?

Yes! Keyword Cannibalization is basically the use of same keywords in multiple blog posts/pages. It makes those two pages compete with each other in SERP. If you are taking special efforts for SEO, this may sabotage them pretty bad. There are different types of keyword Cannibalization

Sub-domain conflicts
Internal Keyword Cannibalization
International Cannibalization
Semantic Flux

Keyword Cannibalization is one of the most common SEO issues and you need to take a lot of efforts to counter it.

No, Its not actually Keyword Cannibalization.

Every individual page has different keyword density. Having a bunch of keywords is not a big issue. But having keywords which have very close meaning, might create a problem at the time of web page crawling.

Basically, keyword cannibalization is bad when you have similar pages (~duplicate content) that rank for the same keyword. If you have categories on your website then the situation is different.