at-most-once delivery
Each message arrives once or not at all; nothing is ever repeated.
At-most-once delivery sends each message a single time without waiting for confirmation, so a loss is never repaired and a duplicate never occurs.
Examples
A sender that never resends loses reading 3, which went into a dead connection at 30 s, but never duplicates anything.
Reading 2 arrived before the link failed; its ack was lost, but an at-most-once sender doesn't care and moves on.