Why will my unit not communicate on certain IP addresses?

I had a Hardy unit hooked up to my network and was able to get online with the browser.  I then removed this unit and placed another Hardy unit in its place using the same IP address.  However, this second unit would not allow me to get online with it, but if I changed to another IP address it works find.
Why would the unit not work with one address, even if a different unit works with that address, but would work with a new address?

The Hardy units that use an Ethernet address are the HI 3000 series, which include the HI 3010, HI 3030, and the HI 4000 series, which include the HI 4050 and HI 4060 units. 
The problem is that Ethernet devices, like your computer for example, remembers the MAC ID address of the instruments they talk to. Each HI 3000 is going to have a different MAC ID address. If you have been talking to one unit at some IP address, and then disconnect it and connect a different unit with the same IP address, the computer is going to get confused because it has associated that IP address with the MAC address of a different unit.

This process of associating IP addresses with MAC addresses is known as ARP (address resolution protocol). You can view the ARP table your computer is using by opening up the DOS prompt and giving the command "ARP -a"

Having done that, you can delete an element of the ARP by giving a command like "APR -d 10.153.31.74"

Doing that with the duplicate IP address will allow you to forget the old unit and connect to the newly connected unit.

If for whatever reason you cannot use the DOS prompt, rebooting the computer also will let you connect.