Steps to reproduce the issue

Found two problems with updates sites rebuild process:

Problem 1

When we update a language package update (ex: pt_PT) the update server is now changed to https://update.joomla.org/.. (in previous versions was http://)
This will create a new update server (this happens everytime a updated site URL is changed in the manifest file) for this "extension".
So now we have two update servers for the same language pack.
Even after rebuild the two update servers are still there.

To reproduce:

• Use latest staging or 3.6.0 beta 1
• install pt-PT language pack 3.5.1.1 from URL (http://joomlacode.org/gf/download/frsrelease/20263/163106/pt-PT_joomla_lang_full_3.5.1v1.zip)
• Now update it to latest version.
• Go to Extensions -> Install -> Update sites you'll see the two update servers for the portuguese language pack.
• Press "Rebuild" and you'll still see the two update servers.

Problem 2

If you change the joomla update server channel in joomla update component options and then go rebuild the update sites joomla will get two update sites.
Even after rebuild the two update servers are still there.

To reproduce:

• Use latest staging or 3.6.0 beta 1
• Go to Joomla update component options
• Change the update server channel to "Testing" and save
• Go to Extensions -> Install -> Update sites you'll see the core update server is something like https://update.joomla.org/core/test/list_test.xml (the testing channel)
• Press "Rebuild" and you'll have two update servers (one with empty description - because of #10827) for core update server.

Additional info

For both problems, since deleting any update server with "update.joomla.org" in the URL is not allowed (which is the case for both problems) we always stay with this additional update servers.

Expected result

Update servers rebuild with no issues.

Actual result

Described above.

System information (as much as possible)

3.6.0 beta 1

Additional comments

The update rebuild process is processed here: https://github.com/joomla/joomla-cms/blob/staging/administrator/components/com_installer/models/updatesites.php#L219

Any help on solving this would be great.