Cannot get rid of "Warning: Each child in a list should have a unique "key" prop." [duplicate]

Can't get rid of "Warning: Each child in a list should have a unique "key" prop."

 const Course = () => {

    const list = course.parts.map(parts => <li key={course.parts.id}>{parts.name} {parts.exercises}</li>)
    const exerscisesOnly = course.parts.map( x => x.exercises)
    var all = 0;

    for(var i = 0; i < exerscisesOnly.length; i++){
        all = all + exerscisesOnly[i]
    }



    const Header = (props) => {
      console.log(props)
        return (
            <div>
                <h1>{props.course}</h1>
            </div>
        )
    }

I have defined the key inside course.part.map but it doesn't want to work