My understanding is Template_A doesn't have the same list of attributes as of Template_Base. If so, you can right click the element (derived from Template_A) and select "Reset to template".
The following link also very useful, if you are looking for the bulk operation.
Please use PI Builder(Excel Add-in) where you can set the parent template. You can export template to excel and work offline and import back to AF once configuration is done.
Not sure how you can have duplicate Attributes on an element, as attribute names are unique across the template / element.
However, to properly distinct where attribute templates are defined, you can switch PSE to show the attributes grouped by Template. That should clearly show which attributes are defined at what level in the template inheritance.
Which version of the AF Server are you running? I'm going to try and reproduce this in our systems.