Ipod troubles with artist names?

ok well on my ipod i have duplicated artist names but its not like i have two slayers and one has 5 songs while the other has 10. my problem is that ill have 8 different melvins on mine while the first one will have 10 and the second 3 but the rest are the same so its like i added the same songs 8 different times and i dont know why its showing up like that on my ipod when that doesnt happen on itunes. anyone know how to fix this? and one more thing if you know how to make certain songs play on your ipod (as in you add them and it skips it and ont play it) then pease tell me that tooo thanks ^^