Add a key for progress bars.

lib/components/UploadPage.js

@@ -89,7 +89,7 @@ export default class UploadPage extends React.Component {
           {keys.length > 0 ? <p>Download Progress</p> : null}
           <div className="data">
             {keys.map((key) => {
-              return <ProgressBar small value={this.state.peerProgress[key]} />
+              return <ProgressBar small key={key} value={this.state.peerProgress[key]} />
             })}
           </div>