Loops in routers-same packet? Considering a loop topology between a few routers with a host connected to one of them, when it's default routing directs to the cycle. Even when observing Wireshark, how can we know its the same packet and not different ones? The only field that was the same was the ipV4 checksum, which seems to me a bit unintuitive as the header checksum differed. Why wouldn't they be the same? Is there another, better way to deduce that a looped packet is indeed looped and the same packet? A thing I noticed was that the difference between all the TTL's was the cycle's length, but it's not concrete, just a guess from an observer's perspective. And if they are the same, what makes them different apart from TTLs after each cycle?

There is nothing in the IPv4 header that enables you to identify duplicates. A packet is a duplicate when it matches a previous packet in entirety, header (apart from TTL and checksum) and payload, and the source has only send one such packet.

As @manishma has pointed out, you would need to hash each packet, store the hashes for a certain time and check for matches between them. If there's in-path fragmentation you'd even need to reassemble fragments before hashing. Of course, packets with very low TTL values are a clear warning sign.

A routing loop is most easily detected by using traceroute-type probes - a series of packets with TTL increasing from 0 - and then analyzing the sources of the ICMP _time exceeded_ messages.