Note to all, this problem not because of template. Do you remember, when you change the template, there's an option feature from Blogger, Do you want to delete these widget? If you choose keep widget, all your blog widget keep there. This feature is usefull for keeping widget so you don't need to add it one by one.
But the problem is, when your new template has Blog1 widget, and your current blog has Blog2 widget, it will make a double post. Note that Blog1 or Blog2 is post widget, so it will appear double.
So how to fix this double post problem?
Open Edit HTML, then find this code
<b:widget id='Blog1' locked='true' title='Blog Posts' type='Blog'/>
<b:widget id='Blog2' locked='true' title='Blog Posts' type='Blog'/>
Just delete one of them.