The only thing that's worse than a negative account on your credit reports is a negative account that shows up twice. Unfortunately, double entries can happen quite easily in the credit reporting system, and it takes a real eagle eye to scour over the details of your credit reports to make sure your credit score isn't taking any unnecessary hits.
While it seems logical that the credit scoring model shouldn't count the same account twice, account listings that are not "completely" identical could both have a negative impact on your credit score. The second listing might not hurt it quite as much as the first, but it will impact the score.
So keep an eye out for double entries the next time you review your credit reports, and be sure to dispute any inaccurate information through the appropriate credit bureaus. Correcting one double entry could be all the help you need to move your credit scores into the next level.