This is regression. Before I was able to add several audio mixers on panel, but not anymore (4.10.0). You may want to add one mixer for playback and another for microphone level, for instance. Audio mixer is dimmed in Add new items dialog, when one mixer is already added on panel.
http://git.xfce.org/apps/xfce4-mixer/commit/panel-plugin/mixer.desktop.in?id=117ff721b989bcf8cd8e0fbeaa692c14c1ecf5ef So why not set X-XFCE-Unique=false again?
I don't understand patch from comment 2 too. How can I set Master and Microphone volume plugin at same time?
You should be able to select which controls you want or if all else fails, do as suggested in comment #2 and edit the desktop file to remove the Unique option. Closing as WONTFIX
Please note that the bug report is about the panel plugin, not about the mixer dialog launched by xfce4-mixer command. The setting Unique should be removed by default. It is too hard for user to find the right file and change it. And as for comment 2, you meant comment 1. mazarik@gmail.com, if there are two panel plugin's added in panel, one could control Master and other could control Microphone, for instance.
Fair enough. Reopening so that it doesn't disappear from the radar.
This should be fixed in git: commit 7a18841c988edb593127842ec5c919df537f9ce0 Author: Andrzej <ndrwrdck@gmail.com> Date: Sat Apr 12 08:53:09 2014 +0100 Make Unique support optional