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

High traffic periods increase congestion and may cause drops, but do not inherently cause the same unicast frame to be duplicated and retransmitted.

Re-establishing a broken link may trigger a topology change and STP reconvergence, but does not by itself cause duplicate unicast frame transmission.

Upper-layer protocol reliability mechanisms (such as TCP retransmission) operate at Layer 4 and above and do not cause Layer 2 frame duplication in the switch fabric.

In a redundant switched topology without STP (or with STP misconfigured), switching loops allow frames to circulate indefinitely. Because switches forward unknown or broadcast frames out all ports except the one received, multiple copies of the same unicast frame can be generated and retransmitted continuously, degrading network performance.

A dual ring topology is a valid, defined physical layout (as in FDDI or MAN designs) and does not inherently produce duplicate unicast frame transmission on its own.