Im relatevely new with Asp.net MVC3,
I have a form handled with Ajax, like this:
@using (Ajax.BeginForm(\"dtjson\", new AjaxOptions { HttpMethod = \"Post\"
You have included jquery.unobtrusive-ajax.min.js twice once in the layout once in the partial. So your browser executes the js inside twice which will subscribe twice on the form click event that is why doing two POST instead of one.
So you need to remove the jquery.unobtrusive-ajax.min.js from the partial.
Note: If your are using a partial with a layout you don't need to duplicate the js included in the partial because it's already done by the layout. There are some good articles about layouts and partials.