monaco-editor version: 0.X.Y
Latest

Browser:
Firefox 78.0.2 (64-bit)

OS:
macOS 10.5.6

Playground code that reproduces the issue:

1. In Firefox on OSX goto https://vscode-web-test-playground.azurewebsites.net/
2. From the edit menu, click Emoji & Symbols
3. Select an emoji to insert
4. Emoji gets inserted twice

Also reproducible at:

• https://microsoft.github.io/monaco-editor/
• https://microsoft.github.io/monaco-editor/playground.html