<ul><li>The warning 'Each child in a list should have a unique 'key' prop' in React occurs when rendering lists of elements, such as when using the map() function to generate an array of components.</li><li>The key prop is important for efficiency in re-renders, providing stable identity, and avoiding bugs.</li><li>The key should be unique and consistent for each item in the list.</li><li>Common mistakes include using the array index as the key and having non-unique keys.</li></ul>

[React - Learn From Problem] Each child in a list should have a unique 'key' prop

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