Linked duplication not working in 2.6.

I’m following the steps for linked duplication correctly but it isn’t working.

When I select a face on one half of the object and extrude it, the same face on the duplicate half doesn’t move.

Link duplicates are for separate objects. When you edit one object all the linked duplicate objects also change. If I understand your post correctly what you are looking for is the mirror modifier where editing one side of an object does the same to the other side of the same object.

Hope this helps,

Ah okay. So basically what the mirror modifier does is it makes a mirrored copy of the object as it is, and then reflects any changes you make to the original half. Am I correct? Does that include selections as well?