Steps to reproduce the issue

• The issue is for sure not reproducable for everybody. As far as I see it has to do with the performance of the server environment.
• Current nightly. Very little installed and very few entries in the database.
• Go to Menus > Manage.
• Set filter on Administrator to create a backend menu.
• Click button New.
• Enter Title and Alias.
• Field Import a preset: Select Preset Alternative Main Menu.
• Save & Close or just Save

Expected result

• All menu items from preset saved.

Actual result

• I get a message like

Menu was saved but failed to import the preset: The alias 1627514907-04 is already being used by - menu item in the asdfgh menu (remember it may be a trashed item).

(asdfgh is the alias/menutype I entered in this case).

The image shows several tests that resulted in different numbers of menu items but the last one (red arrow) where I have manipulated tha alias creation in MenusHelper.php for separator types before.

I located the error here: https://github.com/joomla/joomla-cms/blob/4.0.0-rc5/administrator/components/com_menus/src/Helper/MenusHelper.php#L509

Looks like microtime(true) is not reliable enough in faster environments to create unique aliases.

My temporary but bad (also a bit lottery ?) fix is

$item->alias = microtime(true) . '-' . uniqid();

System information (as much as possible)

• 10.5.11-MariaDB-1:10.5.11+maria~focal-log
• PHP 8.0.6 (Linux dd51328 5.4.0-80-generic # 90-Ubuntu SMP Fri Jul 9 22:49:44 UTC 2021 x86_64 )

Additional comment

-It is normal that in each additional menu with preset altenate.xml 1 more menu entry is added, but only 1.