weird case for IP duplicated on LAN

[IT Support]

On 02/20/2015 12:17 PM, IT Support wrote:
> Hi eveyone. I'm facing a weird case of IP duplicated on my LAN, ISC 
> DHCPd version 4.1.1 running on Debian, this is the case. For example 
> user A already got an IP address from dhcp server (192.168.0.20)  
> 'till here everything goes ok.  Then user B get connected to the LAN, 
> asking for an IP address to dhcp server, assigning the same IP address 
> to the user A (192.168.0.20) creating an IP address conflict.

     Please someone to throw me a bone here?

Thanks in advance.