Baffled by output from `port dependents'

[Mark Dickinson]

When I type:

$> port dependents bzip2

I get the following:

ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2
ImageMagick depends on bzip2

Why all the repetitions?  I only have one active copy of ImageMagick
installed, and all inactive versions have been uninstalled:

$> port installed | grep ImageMagick
  ImageMagick @6.3.2-9_0+darwin_8 (active)

I can only assume that all these lines refer to previously installed,
and since uninstalled, versions of ImageMagick.  How does my system
know about these previously uninstalled versions?  I can't find any
trace of the previously installed versions on my system.  Is there
some file somewhere that still keeps track of all the previous
versions?  Is there any way to update this file?

Mark