I'm about to hire freelance contractors to do small pieces of my web projects.

Is it standard to allow them to put the project on their portfolio? Say a simple debugging task getting a specific mobile problem fixed in <5 hours.

I'm just concerned as it could potentially dilute my company's portfolio by having it listed by each contractor when in fact the majority of work is done by me. I'm trying to build a portfolio here as an agency.

I think it's a risk for my company since I'm working hard to get clients and the whole point is putting projects on my portfolio.

Am I wrong here?

Is there an standard policy for this? Perhaps a limited policy whereby attribution is required to the agency? Did exactly X for agency Y (link).