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 by editing the object's contents you affect all the instances.
If you need an independent copy (not linked to its origin) apply the "New Object via Copy" command.
For instance, on the screenshot 1, the "Source Object copy" has been created from the "Source Object" by means of the "Layer via Copy" command, while the "New Object via Copy" 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 "Source Object" (by using either the "Edit Smart Object" or the "Replace Contents" function), the changes only affect the linked "Source Object copy" (fig. 2).