Windows 10 Pro, GTX 1060 6 GB
Broken: 2.79b, current 2.79 builds
Short description of error
Alt-left should apply a setting to all selected items (where applicable). This doesn't seem to be the case for the group menu under the duplication setting. Only one of the items gets the change, the others are left untouched.
Exact steps for others to reproduce the error
Create a scene with several empties.
Add Suzanne and place her in a group.
Select all empties and using Alt-Left, change the duplication settings to Group and select the group Suzanne is in, using Alt-left click for the menu actions.
Review the duplication settings for each empty. Only one of the empties will have been changed.