Choose to 'Add Property' to any panel; say Custom core-site.xml
Choose 'Bulk property add mode'
Send keys: 'customkey=customvalue' and choose 'Add'
Select 'Add property' again on the same panel and perform the same steps by trying to add 'customkey=customvalue'.
Upon clicking 'Add' button its expected that a UI error displays as this is an existing property.
But its not shown. There seems to be lot of error in the console while trying to perform this operation.