In a document, you may use multiple instances of a single smart object. To create a "clone", you use either the "Duplicate Layer" or the "Layer via Copy" command. The cloned instances are linked to each other, so that when you edit the object's content it affects all the instances.
If you need an independent copy (not linked to its origin) apply the "New Object via Copy" effect.
For instance, on the screenshot below "Smart Object copy" has been created by using the "Layer via Copy" command, while "Smart Object copy 2" is a new (independent) smart object. As you can see, the two copies seem to be absolutely identical.
Nevertheless, if we alter the contents of the original "Smart Object" (by using either the "Edit Smart Object" or the "Replace Contents" function), the changes only affect "Smart Object copy" (linked duplicate).