In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?

In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?

A.
after broken links are re-established

B.
in an improperly implemented redundant topology

C.
when upper-layer protocols require high reliability

D.
during high traffic periods

E.
when a dual ring topology is in use