Hello Saurabh,

May I ask which version of our controls you are currently using? The behavior you are encountering has been seen previously in version 2013 Volume 1 build 2021. We provided a fix starting in hotfix 2069. I recommend that you install the latest service release for version. I have done some testing and dropped the UltraCalendarCombo using the latest, 2014 Volume 2 and did not see duplicate weeks in December.

Installing the latest service release for Infragistics helps keep your application up to date. 
The latest Service Release build is available at Infragistics.com.

1. Go to https://es.infragistics.com/my-account/keys-and-downloads. You may be prompted to log in with your username and password.

2. You will see a list of all of your registered keys. Select the version of NetAdvantage you are interested in.

3. Select the "Services Releases" tab on the bottom.

4. Click on the item you want to download. 

For general information about service packs, see our Service Release Schedule http://es.infragistics.com/support/service-releases

Let me know if you have any questions regarding this matter.