ID3 tags

in my ipod some names of the artists repeat twice when it's written just the same way, the difference is that in one of them are half of the songs and in the other- the rest of the songs belonging to that artist..
i noticed that the ID3 tags are different for those songs.. so ipod sort them by id3 tags..
so what should i do that all the songs belonging to certain artist show up under that artist and my ipod doesn't write the same artist name 2-ce in his menu...