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[MGallo]

when you open your save file, use mode 'a' (append)
Now you can just add new people to the end of the file.
It would be even better to use json format, which you can load,
append in memory, and rewrite when done.
There are some good tutorials on json
Here's one I like: https://pymotw.com/3/json/

I'll play with your code a bit and get back soon

Thank you so much! :D