Description of problem:
I can add multiple alarms on same time.
This bug has actually 2 closely related sub-bugs.

Version-Release number of selected component (if applicable):
gnome-clocks-3.8.0-1.fc19.x86_64

How reproducible:
always

Steps to Reproduce:
1. Run Clocks
2. Set two alarms on exactly same time.
  
Actual results:
1. You can add second alarm.
2. When time comes, you can disable only one alarm, second keeps beeping.

Expected results:
1. User should be warned that alarm for same time is already setup.
2. In case user sets 2 alarms on same time, he should be able to stop or delay both of them.