In most websites we can see that the cover image alt text is same as post title (e.g. blog archive), meaning that a sentence or keywords are duplicated, Does it hurt SEO? should we define different alt text for images?
Technically, this won't hurt SEO, because the post title and the alt text are nestled into different HTML tags, which are used for different purposes. One is used as the title of the written content and for general search, while the other is for accessibility and image search.
That said, their different purposes usually mean that the post title and the alt text should not be the same. If your CMS simply duplicates the title, you should, in most cases, change the alt text to something descriptive about the picture.
This will help users who search for your keywords to find your image. (Especially true if the cover image is an original and specific one, rather than a random stock photo.) It will also prevent the screen readers from reading the same headline twice. (Screen readers will read the alt text as they go down the page, usually in monotone; imagine how annoying that would be to the user.)