Hello,
Please disown the package xfce-theme-greybird-git[0], as it has been flagged out-of-date since 2014-03-17 and I emailed to the maintainer on 2014-05-07 but still get no response. I think the PKGBUILD should be updated to at least v1.3.4 for fixing the dual-titlebar/shadow issue[1].

[0]: https://aur.archlinux.org/packages/xfce-theme-greybird-git
[1]: https://wiki.archlinux.org/index.php/GTK%2B#Client-side_decorations

Izumi Natsuka