Remove duplicate properties in lessc compileProp()

Inserted a new line in lessc.inc.php (line 711) to remove duplicate properties.  This should only remove properties in the same block that have the exact same values.

This ought to fix #233, where multiple imports of the same mixins would cause duplicate properties...  There may be a better way to fix this, but I'm not sure where it would happen.  This will also result in generally cleaner, more compact CSS.