[ACD] A received ARP probe (having the target IP address set to the device's IP address) must be treated as conflict during probing phase.

RFC 5227 chapter 2.1.1 "Probe Details":

[...]In addition, if during this period the host receives any ARP Probe
   where the packet's 'target IP address' is the address being probed
   for, and the packet's 'sender hardware address' is not the hardware
   address of any of the host's interfaces, then the host SHOULD
   similarly treat this as an address conflict and signal an error to
   the configuring agent as above.  This can occur if two (or more)
   hosts have, for whatever reason, been inadvertently configured with
   the same address, and both are simultaneously in the process of
   probing that address to see if it can safely be used.

If a probe is received that has the target IP address set to the IP address that is just being probed by the lwIP stack, then this shall be treated as a conflict.