Following method works pretty good for me:
- open your form;
- use the filter (one of the icons on the bottom right of the form window) to select the existing record to duplicate (or to select a small set of records including the intended one);
- activate an additional table view (one of the icons on the bottom right of the form window): the selected record(s) appear(s) on top of your form;
- simply select the row you want to duplicate (click the green arrow left of the row) and drag it to the bottom of the additional table view (the bottom of this table view contains an empty row marked with the yellow star icon "Insert a new record").
NB: opening the additional table view without selecting records, and trying to duplicate directly last row of the table does not work, presumably because of to many records. After selection of records, it works every time for me. I don't know if there is a maximum number of records to allow in the selection.
Hope this helps,
Paul