>> Because Gluu server is slow and the "Login" button is still available during several seconds after a first click.
No one complained about 'slowness' of Gluu Server; and double login button clicking won't help you to make it faster. May be you can use a better environment which has better I/O capability. Pushing login button twice ( quickly ) will push tomcat to throw error on 'Concurrent Call'.
If your Gluu Server is slow,
1. Try to use better VM.
2. Check oxauth.log why it's taking longer time to complete the authentication.