Google accounts for these sort of "boilerplate" content blocks.
In this case you're talking about like a Call to Action statement.
You're totally fine, it's not going to hurt you. These are very normal sections for pages to contain after the primary body content is finished.
What would be duplicate content?
If you copy and pasted content from ...
Imagine what each page would look like if you removed the 300-word advert, and if that hypothetical page would still be unique, fleshed out, and stand on its own as useful content, then I would not worry about your pages being downranked as duplicate.
Also note that duplicate content does not count towards any penalties, unless it is blatantly excessive or ...