In the main application, possibly under "Manage >> Options", add an option to "Edit on Duplicate". When this option is checked, it will change the "Duplicate Entry" behavior. Adding a checkbox allows those who may prefer the current behavior to continue using it.
When "Edit on Duplicate" is not checked, behavior is not changed. To Duplicate an entry, a new entry with " Copy # x" in the title is created which can then be edited.
When "Edit on Duplicate" is checked, behavior is changed. When "Duplicate Entry" is chosen, instead of creating a new entry with a different title, a "Duplicate Entry" dialog box, similar to "Add Entry", is openend with pre-populated fields/data including password history. And, if it's not too difficult, as each field is modified, bold the field name to indicate that field value has been modified. And in addition to the "Ok, Cancel, Help" buttons, add a "Reset" button which resets the fields to the entry being duplicated.