At {11:18] what did you mean by them having the same mesh data?
They are duplicates sure but i thought if 2 objects had similar data they would both change when edited.
You're thinking of linked mesh data. Duplicates have the same (matching) mesh data but don't necessarily share a connection, meaning you can change one without affecting the other. Linking mesh data is the connection where both are affected by changes made to one or the other.
Matching mesh data is required for many operations in Blender like transferring UVs, vertex groups, shapekeys, multires reshape, etc between mesh objects. Matching mesh data is also required to utilize linking as well.
Does that make sense? I know this can be confusing to read textually..