I have some linked duplicates I am working on.
If I edit one of them, I have to tab into Object Mode to see the other instances get the changes I´ve made.
Is it possible to see the other instances update at the same time as I edit one of them?
You probably don’t actually have linked instances if you are not seeing edits happen to all objects at the same time.
Review the data mesh context and make sure you see a number next to the name of the datablock. If there is not a number then you don’t have instances. The number represents the number of objects sharing the datablock.
Thanks for your reply Atom!
It is linked instances, but it seems to be a problem that I have modifiers. It worked when I applied them, so I can get it to work almost as I wanted 
Yes, it’s a performance issue. While objects share the actual mesh, the geometry generated by modifiers is not shared. That means that for realtime updates Blender would have to constantly recalculate all modifiers on all instances while you’re editing, which can get pretty slow. So instead they update when you “commit” your edits, that is, leave edit mode.
Thanks for the clarification