What is duplicate ACK when does it occur?

I’m currently learning about TCP protocol and don’t understand what duplicate acknowledgement are when they will occur. Also doesn’t retransmission only happen when a ACK is not received from the receiver to the client? To work out a retransmission you have to add the round trip time (from 3 way handshake) by the safety margin?

It will be very helpful if you can explain these to me with an example perhaps as I am a beginner to this.

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