Fix duplicate collaborator entries in collaboration panel

[nzinfo]

Problem

When the same user connects from multiple browsers, the collaborator panel may show duplicate entries even when they are viewing the same file. This causes unnecessary clutter and React key warnings.

Solution

• Add deduplication logic in _onAwarenessChanged using a Set to track unique combinations of username and current file
• Only add a collaborator to the list if their unique key (username + current file) hasn't been seen before
• Use consistent unique keys in both the awareness change handler and React component rendering

Changes

• Add collaborators_keys Set to track unique collaborator entries
• Create unique keys using username and current file path
• Filter out duplicate entries before adding to collaborators list
• Update React key prop to use the same unique key format

Testing

1. Open the same notebook in multiple browser windows with the same user
2. Verify only one entry shows per unique (username, current file) combination
3. Check browser console - no more React key warnings

[github-actions]

👈 Launch a Binder on branch nzinfo/jupyter-collaboration/fix_react_warning

[nzinfo]

The warning

Warning: Each child in a list should have a unique "key" prop.

Check the render method of CollaboratorsBody. See https://reactjs.org/link/warning-keys for more information.
Collaborator@http://localhost:8888/lab/extensions/@jupyter/collaboration-extension/static/collaboration_lib_index_js.0471f9a278b7578b8646.js:105:76
CollaboratorsBody@http://localhost:8888/lab/extensions/@jupyter/collaboration-extension/static/collaboration_lib_index_js.0471f9a278b7578b8646.js:96:94 react.development.js:209
React 5
printWarning
error
validateExplicitKey
validateChildKeys
createElementWithValidation
CollaboratorsBody collaboratorspanel.js:75

[davidbrochart]

Thanks for the PR @nzinfo.
How about making collaborators a Map, instead of having the keys in collaborators_keys and the values in collaborators?