Wouldn't Parker meet himself when he backstepped, thus effectively making two of him? To this troper's knowledge, meeting himself in the past was never an issue nor was it explained away.
We're led to believe that the instant a Parker (and the Sphere) arrive from the future, the version currently existing disappears into thin air, as a correlary to the "a person can't exist in the same place twice" theory of time travel. Thus the instant the Sphere disappears, they just wait for Parker to call.
This is backed up in one episode where the Sphere leaves without Parker. We skip to the past where Donovan and Ramsey are going "yeah, wonder where Parker is this time", and are surprised when Parker appears right behind them, eating chips, asking what's going on. He doesn't know, since he's never at the base when they do the "sphere's gone, something bad happened" thing.