Question

Does TCP/IP prevent multiple copies of the same packet from reaching the destination? Or is it up to the endpoint to layer idempotency logic above it?

Please reference specific paragraphs from the TCP/IP specification if possible.

Answer 1

It’s the TCP stack’s job to recover from duplicate packets:

The TCP must recover from data that is damaged, lost, duplicated, or delivered out of order by the internet communication system. This is achieved by assigning a sequence number to each octet transmitted, and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. At the receiver, the sequence numbers are used to correctly order segments that may be received out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments.

— RFC 793 – Transmission Control Protocol, Section 1.5

However, if they’re the same packets with new sequence numbers, then no.