I just tried the following:

Using the classic tracker in demo.py added:

                  due_date=Date()

to the end of the IssueClass("issue") definition.

then in issue.item.html I added:

<tr>
  <th i18n:translate="">Due Date</th>
     <td tal:content="structure
python:context.due_date.field(format='%Y-%m-%d')" />
</tr>

right before the Change Note.

If I click on "Make a copy" which I assume is the same as your
"duplicate issue" I get a perfectly good copy of the issue with all
fields copied.

I am closing this as works for me. If you can tell me how to reproduce
this in a demo.py tracker I will investigate further.

-- rouilj