Why is Shift+D Duplicate creating linked duplicates? So I've been learning Blender for the last week or so and have been having a blast! 

I did stumble onto a very frustrating problem though. Whenever I duplicate an object using Shift+D, it creates a linked duplicate as if I had used the command Alt+D instead. I first noticed this during a tutorial where the presenter duplicated an object several times and then rotated each one individually. Mine would all rotate together. I thought I might have enabled an option accidentally while following tutorials so I tested this in a brand new scene with just a cube. Sure enough after making a couple of duplicates (Shift+D), any changes made to the original were instantly copied to the duplicates.

I'm not sure if I'm missing something very obvious here, I don't think that is the normal behavior for the Shift+D function. I'm kind of at a loss. Any help would be greatly appreciated!
So I discovered what the problems was. In the User Preferences > Editing tab, the Mesh option was unchecked under Duplicate Data. After enabling it the Shift+D duplicate functions normally. I unchecked it again just to make sure that it was indeed what was causing my issue.