Do not allow to have identical calendars
Details: The current Gnome calendar built from the master allows us to have the same calendars, which means that we can have calendars with the same name and color.
Expected result: Don't allow to have this sort of stuff.
Steps to reproduce: Just create two identical calendars, and you will get things messed up.
Link to original bug (#781284)
Design Tasks
TODO
Development Tasks
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QA Tasks
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