Q: Is a formal withdrawal needed if I don't want to resubmit to the same journal?
A decision letter that has come several months after submission mentioned that resubmission should be made by 6th Dec. However, I would like to submit my paper to a higher IF journal. What should I do? Is it okay to send email to the journal about withdrawal? Is it duplicate submission if I don't send my widthwal letter but submit my revised paper to another journal? If I submit my paper after 6th Dec is it duplicate submission?
You haven't clearly mentioned whether the decision was a "reject" with the provision for resubmission or a "revise and resubmit." In the former situation you can choose not to resubmit and can go ahead and submit your paper to another journal without sending a withdrawal email to the editor. However, if it was a "revise and resubmit" decision, the withdrawal procedure would need to be completed before you can submit the paper to another journal.
Even if you intend to submit your paper to the second journal after December 06, it would be advisable to inform the editor of the previous journal that you are not going to resubmit your paper. However, it would be unethical to withdraw your manuscript just because you want to submit it to a higher IF journal, since the reviewers have already spent so much of their valuable time on your manuscript. Moreover, if you submit to another journal now, you will end up losing more time in the review process. Also, you cannot be sure that your paper will be accepted by a higher IF journal. If the paper is rejected, you will end up losing more time and will have to repeat the process all over again.
What is more, the impact factor is not a true measure of an individual article's impact, and the academic community is trying to move away from the overemphasis on the impact factor. If the current journal is a reputable one and has shown interest in your paper, I feel it would be wiser to consider resubmitting to the same journal.