Concurrent creation of pages/components works only for the first user

Steps to reproduce

1. Log concurrently in Magnolia  Admincentral with to different sessions (for example, using two different browsers). https://demo.magnolia-cms.com/.magnolia/admincentral#app:pages-app:browser;/travel/::
2. Click create a page
3. Select Basic template. 
4. When using different page names in different browsers, we hit an issue caused by PAGES-271. When using the same name, the issue is reproducible only on root, the other levels throw a repository exception.
5. Click next.
6. The ¡Page properties' dialog is opened, (optionally fill out the info) but do not save it yet.
7. Go to the other browser, repeat steps 2], 3], 4], 5], 6]
8. Save the dialog.
9. Page is created in this browser.
10. Go back to the first browser and save the 'Page properties' dialog.  

Expected results

Page is saved in JCR (shouldn't this fail with a page already exist message?)

Actual results

The dialog is closed, there is no new page created in Pages app, and there is no error.

Workaround

Development notes