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According to Tanenbaum's Modern operating systems,

The purpose of this field is to detect replay attacks. These sequence numbers may not wrap around. If all $2^{32}$ are exhausted, a new security association must be established to continue communication.

According to Forouzan's Data communications and networking,

In both protocols, replay attacks are prevented by using sequence numbers and a sliding receiver window. Each IPsec header contains a unique sequence number when the security association (SA) is established. It ranges from $0$ to $2^{32}-1$. When the sequence number reaches its maximum, it is reset to 0, the old SA is deleted, and a new one is established. To prevent duplicate packets, IPsec mandates the use of a fixed size window at receiver.

Can someone elaborate this with an example?

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