Since there are numerous fields, but only certain fields values are different, then i am thinking to set the default value to all the fields, so that users just need to alter those values when needed.
In my approach, i will hook a javascript function on load of the form data entry screen and use XmlHttp approach/Ajax approach to hook to the custom web service to pull/retrieve the default values of each fields. Or you can set those values at the javascript function itself, but the drawback of this, it's difficult to customize later. So i will choose the approach to hook to the custom web service and retrieve those value from some application parameter entity.
Your idea of providing a "clone" button is also a great idea, which means that it will duplicate all the attributes of the previous record, into a new record, so that it will save time for data entry person to customize the different value
EDIT
Since you would enter records in batch mode, how about customizing .ASPX screen to enter records. By customizing through .ASPX screen, you can use a tab , so that users can browse through tabs, to customize the value/attribute of each record.
There will be a "save" button as well as "clone" button to clone some common attribute or value.