what command are you using to install your self compiled ipkg, and what's the filename??
The "official" optware repository also has hello_2.3-1_mipsel.ipk, so if your self compiled version has the same name (version), but a different md5-checksum, you get that error.
So either give your own ipkg another versionnumber (i.e. 2.3-2) or just use the version from the repository (cause it's the same anyways).
(a third option would be, to run "make index" on your crosscompile machine, make the packages directory available via http, and change the /opt/etc/ipkg.conf to point to your own repository.
then run a ipkg update, so it gets a list of your own packages (+ md5-checksums). After this you can install your own packages.)