You should put the key on the outer element:

const peopleCard = this.state.people.map(person => (
  <Col key={person.id} sm="4">
    <PeopleCard person={person} />
                                            
sometime it still give issue if your two key values get match so better use this way.
 render() {
let peopleCard = this.state.people.map((person, index) => {
  return (
    <Col sm="4" key={`${person.id}+${index}`}>
      <PeopleCard key={person.id} person={person} />
return (
  <Container fluid>
    <Row>{peopleCard}</Row>
  </Container>
were no two loop will have same id with index which can make key value different.
                                        
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