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I'm reading the RFC 793.
https://www.ietf.org/rfc/rfc793.txt
The acknowledgment mechanism employed is cumulative so that an acknowledgment of sequence
number X indicates that all octets up to but not including X have been
received. This mechanism allows for straight-forward duplicate
detection in the presence of retransmission.
I am stuck here. I am wondering how is that possible?
You need to read the whole section - the current version of TCP is in RFC 9293 (emphasis mine):
3.4. Sequence Numbers
A fundamental notion in the design is that every octet of data sent over a TCP connection has a sequence number. Since every octet is sequenced, each of them can be acknowledged. The acknowledgment mechanism employed is cumulative so that an acknowledgment of sequence number X indicates that all octets up to but not including X have been received. This mechanism allows for straightforward duplicate detection in the presence of retransmission.
"This mechanism" refers to the sequence numbering, not necessarily to cumulative acknowledgement. Without sequence numbers, you'd be hard pressed to detect duplicate transmissions.
