Description of problem:
For an HBAC rule, you can choose to add a Host in "Accessing" section. Then do a find, find it, and add it again to the list of prospective members. It will throw an error when finally clicking on Enroll...but it shouldn't allow to find the host for available list, when it is already in prospective list. 
Same behaviour seen in other areas where adder dialog used.

Version-Release number of selected component (if applicable):
ipa-server-2.1.1-4.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1. Add and edit HBAC rule
2. Go to its Accessing section, and click to Add a Host
3. checkbox a host (previously added), and click on >>
4. In the 'Find' field, search for this host, and it will be listed in Available section
5. Choose it, and click on 'Enroll' again


Actual results:
This can go on, and same host can be added multiple times to the list.
Click on 'Enroll' and error is thrown, but this host does get enrolled  - once.

Expected results:
A host in Prospective list should not be 'found' in Available list


Additional info: