There is a usually a very fine line between a well-wishing colleague and an exploiting colleague. And most of the time, the roles interchange. The truth is, its in the right of a manager inherently to exploit a team mate. Thats how he will be able to get the best out of his men. Thats what he is there for, he is the leader and he leads by showing the path.
But the rules change when your colleagues/equivalents try to exploit you. Now, if your colleague is concentrated towards productivity, its good that you deal and share your work with him because you end up completing your projects on hand quite quickly. You grow, you help him grow, you get the work done and you acquire reputation. But..... if your colleague is an exploiter, if his real intention is to "completely" take over the project from you, finish it (its usually easy to complete a project thats past the design phase) and gain the entire merit for himself, then you better be careful.
Politely decline, saying either that you are not completely through with the design part or that you believe that its a single person's job. Maintain the relationship, because you never know when you might require that person's help in the future (deserately, that is
). But protect your own integrity by standing firm on your decision.