This message means that the file name you are importing already exists in Windchill. Here are a few things to try:
First:
Synchronize and refresh the workspace. Make sure everything is up to date.
Upload - if this fails so will your check in. I use this as a test.
Here are your options.
Search Windchill for that file name. If you get a match and the geometry is the same, you can just replace the version in your workspace with the one from commonspace.
Rename the part in your workspace to something else unique. This will allow you to check in the file.
Rename the existing part in commonspace and keep the file name the same in the workspace. You need to know what you're doing with this option.
