When You “Have an IP” but the Network Still Doesn’t Work: A DHCP/IP-Conflict Fix

I had a wierd situation recently, could get the ip address, set up apt.conf and proxy correctly, but could not connect network, could ping a inner desttop, then my friend found that it responses twice when ping this ip, so it probably is that the ip is used by other people stactly, so I need to get a new automatic ip address, here is what I did,