Not sure, what happened, when i added a custom button for the Mail Merge by following the steps mentioned in those links, it has added the out of the Box Mail Merge button on the form along with the custom button.
To fix this, I have applied the Display rules mentioned in the link to the out of the box button, then removed(UnCustomize button is the menu command in workbench) the custom button that I have added.
It is working as expected for me.
Hope it helps someone.
Regards Jeevan Please vote for me if you find the answer is useful.