I do not think that this is a StackIdeas issue, but we are approaching everyone to see if we can shed some light on this issue.

Issue:
When we try to create a Menu-Item Alias in a new hidden menu to an existing menu item from our Main Menu, then Joomla says that we cannot have a duplicate. But this is a Menu-Item Alias Menu Item and should and has always worked before. We still have many of these Menu-Item Alias menu items working perfectly. Why would Joomla not allow the creation of new ones. We tested this on other sites as well.

Then if we try by manually adding the date&time in the Alias field of the Menu-Item Alias menu item, then we get a '404' when trying to browse to the page...

Any ideas or do you know of a bug or changes in Joomla 3.6.5 which could be causing this?

Thanks,