Dual Lan

My linksys router gets and Ip from my ISP which is 66.xx.xx.xx and then my router NAT's that to 10.10.10.x, he could do the same thing more than likely in his college dorm.

Do you see the difference between a college dorm and what you described to me? You are showing me an external IP NATted to an internal IP.

How is he going to NAT an internal IP to another internal IP on the same subnet without causing a conflict?

Why does it have to be the same subnet. NAT is NAT. Why couldn't it NAT an internal IP to another internal subnet? As far as the router is concerned does it really matter?

I'm sure this is done quite often when one has roommates in college and they wish to share the internet connection.

It might work if different subnets are used but here's a quote from RFC 1597

Using multiple IP (sub)nets on the same physical medium has many
pitfalls. We recommend to avoid it unless the operational problems
are well understood and it is proven that all equipment supports this
properly.

Because the college router will never see any of his devices connected to the router, just the router itself. Same reason why my computer with an IP of 192.168.0.1 doesnt conflict with your computer of the same address. Also, being a college, I doubt they use the 192.168.xxx.xxx block anyway, I'm sure they have their own IP range for intranet use.