The first time you link a homepage to a PA Dashboard a record on [pa_tabs] is created, which links the homepage with the pa_tab. When you unlink the tab from the dashboard, the record in the [pa_tabs] table is not deleted, it is considered "unlinked". Steps to reproduce:

1. Navigate to Homepage Administration > Pages > New.
2. Create a new Homepage and click Save.
3. Navigate to Performance Analytics > Dashboards.
4. Open a dashboard and click + to add content to it.
5. From the Or link an existing homepage choice list, select the homepage created in step 2.
6. Click Link this homepage.
7. Click Edit, then click on the down arrow for the newly created tab and select Unlink this tab.
8. Navigate to Homepage Administration > Pages and edit the name of the homepage created in step 2. 
9. Navigate to the dashboard edited in step 4 and try to link the renamed homepage to the existing tab.

Result: The homepage is not found. The homepage is in the choice list labeled Or link an existing tab.