A user reports difficulties accessing certain external web pages, When examining traffic to and from the external domain in full packet captures, you notice many SYNs that have the same sequence…

Insufficient network resources would manifest as packet loss, retransmissions, or latency increases - not as duplicate SYNs with identical sequence numbers but different payloads.

A failure in the full packet capture solution would result in missing or corrupt data, not in the appearance of crafted duplicate packets with varying content.

A misconfigured web filter might block or redirect traffic but would not generate duplicate SYN packets with the same sequence number and differing payloads.

TCP injection attacks work by crafting forged TCP segments that share the same 5-tuple and sequence number as a legitimate session but carry a malicious or altered payload. This allows an attacker to intercept or tamper with a TCP session, and the duplicate SYN with differing payloads is the direct observable artifact of that forged injection attempt.