Remove duplicate face tags

[GoogleCodeExporter]

Picasa detects and allows duplicate face tags. The current picface code seems 
to reconcile this issue when saving as XMP, however the duplicates are still 
present in the .picasa.ini file. Maybe if the user has selected the Picasa 
"Store name tags in photo" option, picface can ensure that no face tags exist 
in the .picasa.ini after the conversion. Needs a bit more research, but I think 
something should be done to reduce or eliminate duplicate person tags for the 
same person in an image.

Original issue reported on code.google.com by paulw...@gmail.com on 8 Jan 2012 at 4:42