If you're a job seeker and you work with multiple recruiters or staffing / consulting companies, you have to be very aware of who is showing your resume to which contacts. If your resume shows up twice at the same company via two different channels, feathers are going to get ruffled -- this is known as double posting or double submission, and it'll cause recruiters and employers to drop you in a heartbeat.
My first experience with a double submission was close to ten years ago, when I received the same resume twice as a hiring manager. Not realizing the goo I was walking into, I interviewed the candidate, keeping the first recruiter abreast of my progress (we ended up passing on they guy). Shortly thereafter, I got a call from a very angry recruiter #2, demanding to know what the hell was going on. Boy, was I blind-sided.
More recently, I saw this happen again, and this time I was better prepared. I saw the problem before we called the candidate in for an interview (not always easy to do if the recruiter / staffing company mangles the resumes before submitting), and got both sources on the phone. I let them know what was going on, whose submission had arrived on my desk first, and asked them to sort out among themselves who was going to go forward. Many employers would not have been that cooperative.
Luckily, the two recruiters knew each other and reached a peaceful arrangement. If there had been even the slightest remaining conflict, I wouldn't have moved forward with this candidate at all.
Now, I find myself on the other side of the table. I'm working with recruiters, and they're trying to get me in to see some clients. I've tried to keep up with who's showing my resume to whom, and I ended up catching one today. A recruiter started telling me about a "Planning Architect" position, and it sounded familiar. A quick spin through SugarCRM while I'm on the phone, and I find where I've already learned about this position.
The letter of the law in these cases turns on who actually puts a resume in front of a client first, and though the first recruiter in this case hadn't actually sent my resume in yet, I was scheduled to meet them to discuss the position, and I felt compelled to wave off the second recruiter.
Close call.
Hopefully, this isn't a problem that you run into too often, but you do need to be aware and diligent to make sure you're not creating or walking into one of these situations.
Double submitting just shows that you are not professional or loyal enough to the one you are applying, and also it adds up confusions that will end up being annoying.