OpenShot can be tricked into importing the same file

1) Ubuntu 11.04
2) PPA
3) 1.4.0 alfa 1

Steps to reproduce:
1) Add an "alternative path" to a media file that can be imported into OpenShot by making a symbolic link to the folder it lies in.
2) Import the file to OpenShot by navigating through the not-symbolic-link folder.
3) Import the file again, but this time by navigating through the symbolic link.
4 ) You should see that the same file has been added twice to the project. This can also be done with symbolic links pointing directly to the files.

What should happen:
The program should not allow duplicates of the same file.

Patch attached.