I'm not sure the correct fix, but this worked for me in:
/node_modules/@projectstorm/react-diagrams-defaults/dist/link/DefaultLinkSegmentWidget.js
const key = `${Bottom.key}-top`; // Give the duplicated item a unique key
const Top = React.cloneElement(Bottom, Object.assign({ key }, ... rest of current code
I'm not sure the correct fix, but this worked for me in:
/node_modules/@projectstorm/react-diagrams-defaults/dist/link/DefaultLinkSegmentWidget.js