“duplicate name exits on the network” ?

Running w2k server,I have two sites with one IP address. Both web sites work fine. I have tried to set up two ftp sites with each domain name, one public and one private. No problem with one, but with the second, when I attempt to setup second ftp site, I keep getting a message which says “duplicate name exits on the network”. I have set up both identically. What might be causing this?