at-least-once delivery
Each message is resent until it is confirmed, so it always arrives but may arrive twice.
At-least-once delivery retries a message until the receiver acknowledges it; if the acknowledgement is lost the sender retries a message that already arrived, so the receiver must expect duplicates.
Examples
At 63 s the device resends readings 2 and 3; the server stores 3, and stores 2 a second time because its ack had been lost.
A reading stays in the device's memory until the server acknowledges it, so it arrives even after a 40 s outage.