[quote=@Sloth]Unless the Army sending the dog tags back to his family isn't something they did back then.[/quote]
Dog tags during WWI were used to identify the dead. One was placed in the body's mouth and the other sent to the Quartermaster's office to record the death. Dog tags as sets were usually inherited from war survivors who brought both home with them after being demobilised.

While it's [i]possible[/i] that Benji would have both of his father's dog tags, it's much more likely that he'd only have the Quartermaster's one, while the other would've been buried with his father. Even then, this would only have happened if either someone in the Quartermaster's office or someone from his father's platoon took the special effort to send or bring him the tag, since they weren't traditionally sent as standard practice.

Then again, since only just over 1% of American soldiers participating in WWI were killed, such a monumentally bad piece of luck might have prompted such special treatment from one of the father's war buddies.