1. Download JBDS last-1 nightly build for p2 enabled product
2. Download Update site for last nightly build
3. Install previous nightly build using installer
4. ASSERT that list of Available Software Sites in preferences contains two update sites
5. Open Update dialog and add downloaded update site archive
6. ASSERT there is no new update sites in Available Software Sites preferences
7. Check for updates and install new JBDS version
8. ASSERT there are no new update sites added to preference Available Software Sites

Now after step 3 preferences page looks like screenshot21.png.
After step 5 two more sites are added (see screenshot22.png).
After step 7 even mores sites added depends on what features were updated