by MAC blocking

[Wojciech Puchar]

i use static arp on my network.

all existing computers are set in with arp -f /etc/ethers

and interface has STATICARP option set.

trying to use unused IP address doesn't work - as should

BUT trying to use allocated IP address with MAC out of the list - 
surprisingly works.

more strange - when i ping such computer - i get 2 response (normal+DUP)

how to make it completely work?