Here is a wireshark capture of an ARP request PNG image, I contains the sender MAC inside the ARP packet. The receiving station can derive the MAC from the Ethernet frame. It seems to be redundant. Is there any particular use of separately including the sender MAC address in ARP Request too ?.
The "redundancy" was by design (RFC 826), and can be useful in targeting different layers. In RFC 3927 there's what is known as Gratuitous Address Resolution Protocol (GARP), and in certain circumstances the redundancy, or lack of, plays an important role, especially in troubleshooting and monitoring networking stacks.
Actually it's not rendunancy at all, the MAC (physical, layer 2) and IP (logical, layer 3) addresses are not the same thing. They serve different purposes on different network layers.
On large scale networks it's quite common to observe changes in the MAC/ARP/Source/Dest information, and at times can seem almost incorrect. For example, you might see a host send an ARP request with its own address as the target address. Depending on the exact situation, it might be telling us it's a link up/down event, maybe it's trying update other devices ARP tables, or possibly detecting an ip conflict and moving the ip to another NIC.
I could get into clustering, failovers — the list goes on, although I would end up writing a book trying to explain it all. Hopefully this gives you a bit of insight about the "redundancy" you were questioning. ;-)
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