Bug: The Warning 'Each child in a list should have a unique "key" prop.' Should Provide the Duplicate Key

<!-- Please provide a clear and concise description of what the bug is. Include screenshots if needed. Please test using the latest version of the relevant React packages to make sure your issue has not already been fixed. --> React version: 8.3.0 ## Steps To Reproduce 1. Repeat content with the same key 2. Get a warning (in your browser dev tools console): >Each child in a list should have a unique "key" prop. <!-- Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Issues without reproduction steps or code examples may be immediately closed as not actionable. --> Link to code example: No link needed, it's a single line of code: ``` <ul>{ [1,2,3].map(num => <li key="sameKey">{num}</li> }</ul> ``` <!-- Please provide a CodeSandbox (https://codesandbox.io/s/new), a link to a repository on GitHub, or provide a minimal code example that reproduces the problem. You may provide a screenshot of the application if you think it is relevant to your bug report. Here are some tips for providing a minimal example: https://stackoverflow.com/help/mcve. --> ## The current behavior  ## The expected behavior The warning essentially says "You've got duplicate values". But, React knows exactly what key it saw duplicated, right? That key (eg. `"sameKey"` in my example), would be useful for debugging purposes, so why not report it? >Each child in a list should have a unique "key" prop, but the key "sameKey" was seen at least twice.