I haven't read anyone online who quite shares my interpretation of the last scene (spoiler alert, of course). I'd like to know what other folks think.
Rhoda2 is there because on Earth2 she does NOT give her ticket to John2. Everything else is the same (on Earth2 she killed his family, cleaned his house, won the contest, etc.)
For me, this revealed the message of the film. The difference between two of our potential realities is NOT that tragic stuff (e.g. car wreck) happens in one and not in the other. Tragedy happens in both. Where realities split is the moment we have a real moral choice. Does she give the ticket, out of love and hope, or use it herself, out of selfishness and escapism? Our realities bifurcate at such a moment.
In the last scene she sees the version of herself that made the other choice.