Two clients with same IP Address behind a captive portal?

I'm trying to build up a box that acts like a transparent proxy
(similar to SQUID) and a captive portal.

I would like to use a portal like Chilliportal, but i have a question
that i think it's more linux-stack-related than captive-portal related;
the problem is:

I would like that clients with both DHCP IP address and STATIC IP
address can use this system.

So that:
- When a DHCP CLIENT connects:
- He receives the IP ADDRESS
- He tries to connect to "www.google.com"
- The captive portal catch che connection and redirect the user to
an html page for authentication
- and so on...
- When a STATIC IP CLIENT connects:
- My box replies to every ARP REQUEST sent by the client in search
for his gateway
- The client tries to connect to "www.google.com"
- The captive portal catch che connection and redirect the user to
an html page for authentication
- and so on...

But here are the troubles. What happens if two STATIC IP CLIENTS have
the same address?

Let's assume that their own stacks will never collide: i can separate
them with VLAN switches, so there's no problem, they will never see
each other traffic, and they even don't know that there are two
identical IP ADDRESSES on the same network.
The problem is on the linux stack of my box: what happens to the ARP
tables ? The ARP daemon sees two different MAC ADDRESS under the same
IP address. And the NAT daemon will NAT back the connections in the
wrong way.

I was thinking about a solution like this:
There are two different daemons:
- The DHCP daemon: acts like a normal DHCP daemon
- The S(tatic)HCP daemon: when a packet is received from an IP ADDRESS
that is not in the DHCP lease list, the IP SOURCE ADDRESS of the packet
entering the stack is changed, and the box assigns a new address taken
from a "SHCP lease list", assigning him the corresponding MAC ADDRESS.
So that the rest of the chain will never see two address colliding, and
the NAT can work with the new IP address.

What do u think about this? How can this be realized? There is a
simpler way to perform this task?

Thank you in advance,
Marco