Hi!
I'd like to suggest to add and option --omit-dir-times to duplicity similar to analogous option in rsync. Here's the excerpt from rsync man:
-O, --omit-dir-times
This tells rsync to omit directories when it is preserving modification times (see --times).
The directories times often change on creating/moving/deleting files in them (especially in home dirs). But personally I rarely wish to store this information in backup archives.