Hi Kimball,
Sounds like you have attempted to import a package that your target model thinks already exists.
For the quick answer look at the second last paragraph now. If you want to know what happened and why, read on.
EA identifies each package (and pretty much everything) by a GUID (Globally Unique IDentifier). When you create a new project, you have the option of using the ones from your original template project, or causing EA to set new ones.
When EA imports a package, it checks the new package GUID against the target package's GUID. If they are the same, EA assumes you are updating the target package, and replaces it with the imported one. If not, EA generally creates a new package.
However, if the new package's GUID already exists in your model, EA assumes that some kind of mistake is happening, since it seems that you already have the new package elsewhere in the model. Rather than allow you to corrupt the model, ea flags an error. This could happen if you export a package from one location in the model and try to import it into another, without first removing the original.
Of course, you might just want to copy the package to a second location in the model. In order to do this, you have to use a new GUID for the second copy of the package (the one you are importing). You do this by checking the Strip GUIDs box on the Import dialog. This assigns a new GUID to each element (etc.) during the import, and the problem goes away.
The reason you might want to keep the original GUID is replication. If you share models or parts thereof, EA has to know whether an import represents an updated package (or a tree of packages). In this case the intent is to replace the current version with a new one. This can allow you to partition a model and work on separate pieces, perhaps with several people.
HTH,
David
