In short:
D2 in line selection mode is handled by a Pro Addon script BreakOutPro.js.
It calls the standard script BreakOut.js for each crossed entity ...
... And then again for each intersection with the auxiliary line.
Creating a fully new trimmed copy each time. Deleting the original.

For closed shapes resulting in duplicates.

For open shapes with more intersections the outcome varies.
Not all are duplicates, some are partial duplicates.
Per definition not always including the intended results.

Also:
Please close FS#1983
D2 in line mode is kinda a solution I use these days.
One of the many trailing open reports.

Regards,
CVH