Sometimes you may get the error notice as
Another menu item with the same parent has this alias Error building Admin Menus
when trying to install Joomla extensions,the extension is installed succesfully but could not find the admin menu.
Here is the solution:
Firstly,uncompress the extension and open the manifest(xml file),find the menu label under <administration> as (just for example)
<menu img="..EXTENSION.png" view="items">COM_EXTENSION</menu>
remember COM_EXTENSION here.
Secondly,Login to your database,then go to the table #__menu,and try to find the line where alias named as "COM_EXTENSION" then delete.
Finally,install the extension again and every thing will be right!