The program distinguishes computers by their network addresses.
When you are scanning the network by an IP address range and do not resolve the resulting IP addresses to DNS names on the final step of the Scanning Wizard (this is done automatically in the latest versions), the program will store the IP addresses in the "Network address" field for each found PC.
If you import computers from the AD or Network Neighborhood, the program will remember them with DNS and WINS addresses in the "Network address" field. If these computers were already added while the IP address scanning and their addresses were recorded as IP addresses, the program will consider these computers as new PCs because of different "Network address" fields. This will cause to the duplicated PCs creation.
To prevent the duplicate creation, use the same addressing every time you add computers to the program. These can be: 1) IP addresses only (if they are not fixed) or 2) DNS/WINS addresses only (this variant is highly recommended when DHCP is used on the network).
The Client application fills in the "Network Address" field on a computer where it is running by retrieving the computer's address from the operating system locally. If you scan this PC via WMI remotely, you can use some different address format and not exactly the same address string. Again, this can lead to the duplicate creation.
Please use the "Data -> Find duplicate PCs" function to find and delete duplicated computers.