If you are on an ADSL connection /localinfo -u and -h should get different IP addresses in almost all cases. Because most ADSL modems are in fact special purpose routers, that are conncted to your PC through a LAN connection. And some of them are highly configurable. Maybe something is wrong about the settings of your modem, then refer to its manual or contact the manufacturer for more info.
Which IRC network are you connecting to? Have you tried /dcc sending on other networks to other persons? Please try doing it on a network that doesn't mask your IP/hostname. The receiving user should see a line like this:
Ozzielee DCC Send file.jpg (You IP here)
ask him/her to copy/paste that line to you and compare it to your IP to see if it is correct.
And finally, (although this rarely happens), some ISPs force their users to connect to internet through a proxy and assign the same IP address to multiple users. It has been reported that those users are having difficulties sending files through dcc while they can receive files without a problem.