Is this true?
Based on my experience as a hiring manager - yes itsit's unprofessional.
I would definitely not allow a recruiter to present my credentials to a company I am already working with. In the past, it was annoying to have a candidate presented by a recruiter and then already be in our system via their own efforts. It came across as desperate.
Another point to consider is if the company has to pay a recruiter, that is potentially less salary to pay you.
The only exception I would make to this is if you discovered that you knew someone who worked for the company already, then perhaps they could present you to the hiring manager, and up your chances at getting your foot in the door. And even in this case, it isn't a recruiter.