i upgraded fc2 to fc3 with rpm -Uvh,but now when i checked there is many duplicate packages remained from fc2,which is defficult to remove manaully,plz tell me if there is any command to remove the old packages?
Unfortunately no. The rpm command doesn't understand the concept of obsoleted packages, which is why upgrading via rpm is not supported. The yum command can perform this function, but once you have upgraded the way you did, your system is a mess.
I suggest backing up your data, and performing a clean (format/partition) install of FC3.