Description of problem: rpmlint doesn't detect duplicate files if given a directory of RPMs to check. Version-Release number of selected component (if applicable): rpmlint-0.84-2.fc9 In short, if something like $ rpm -qlp *rpm | sort | uniq -d produces any output, then rpmlint should treat is as an error.
The unit of work of rpmlint is a single package (apart from some very specific exceptions with installed packages) and I'm not aware of any efforts to change that. Also, I think that implementing this RFE as suggested would not be too useful - the vast majority of cases it'd flag would be non-issues, for example "duplicate" files in same package for different arches, or same package but different EVR, or otherwise files intentionally owned by multiple packages. And if implemented anyway, I think a warning instead of an error would be more appropriate. If you disagree, I suggest opening a RFE at rpmlint.zarb.org to see if some other upstream rpmlint maintainers are more receptive to the idea.