In the world of contingent search, it is not uncommon for different search firms to have the same listing. The last thing you want to do is have two different recruiters submit your name to the same employer. To most of us, the reasons are obvious (companies are not going to appreciate getting the same resume from different search firms.) But being honest with both recruiters is also critical since at least one of them may be representing you to potential employers. Would you want an agent to feel like they are being jerked around?
One little known fact is that you are under no obligation to work with the first search firm that tells you about an opening. You get to decide who will best represent your interests.