Penny's Horrible Fate (Spoilers)

I'm not sure if this has been brought up before, but I was watching this the other day and something occurred to me. Mr. Peabody and Sherman go back to the past to rescue Penny from marrying King Tut and in all likelihood being killed a short time later when he dies. What follows is a series of adventures during which Penny and Sherman believe Mr. Peabody to be dead. Their solution is to go back to the future to the moment just before Mr. Peabody and Sherman left to go back to ancient Egypt to rescue Penny in the first place. When the duplicates touch they merge into what I can only assume is an amalgam of the past and present Mr. Peabody and Sherman.

But here's the problem: Penny never merged with her double, she's still trapped in ancient Egypt. They never traveled back to rescue her since they had to deal with time falling apart. And they were hardly going to go back after everything was resolved and risk causing another world-ending paradox when she touched her double.

So that means there is a version of Penny that was left to die (brutally) in the past.

I know this is just a kids movie and not a dissertation on temporal mechanics, but it strikes me as a pretty obvious plot hole.