Closed Issue: Setters need to check value is different before executing the setting code and triggering change events. IsUpdating needs to be consistently taken into account. [13399]

This means that for costly scenarios like dynamic poplation of content either on the fly(from js cache) or from a web service, the data is retrieved/computations performed.

Why don't you handle value with set_activeTabIndex : function(value) and bailout if the value matches the current index?

Or you may provide a feature to turn this off at the control level
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http://forums.asp.net/t/1169218.aspx