score:2

you need to provide the key props to the reactcsstransitiongroup component not the child li tag. also using an index parameter in your map function to populate the key.

something like this will work

peoplelist.map(function(person, index){
    return(
        <reactcsstransitiongroup
          key={index}
          transitionname="example"
          transitionappear={true} transitionappeartimeout={1000}
          transitionenter={true}
          transitionentertimeout={1000}
          transitionleave={true}
          transitionleavetimeout={1000}
         >

        <li key={index}>{person.name}, {person.age}</li>
    )
}

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