This is not proper English. We say that we want tomato soup, period. If we want to use "a" then we would say that we want a bowl of tomato soup.
'I would like' is listed as possible meaning of 'voglio', and yet was marked 'wrong' when I used it in my answer. It's not the literally correct translation of 'voglio', but it is natural to say it in this context; presumably that's why it is listed!
"I want" used to be considered impolite. Hence the saying: "I want never gets". "I would like" is more likely to get you good service!
Asking Duo to correct something here is useless. If you report it properly it may get done. Have you read the pop up that appears when you come to comments page? See peter2108's post. It's ok now.
It's acceptable colloquial usage just as we say "I want a coffee". instead of "a cup of coffee" but it is a rather gray area.
I suggested (2014-01-19) they accept I want tomato soup since a tomato soup is pretty marginal usage.
Now, that's good to hear. Probably they'll accept both but as long as the most common is included we sound better.
Belianis. you are right. In italian "i want" does not exist. They use the polite form "i would want". It is a subjuntive. That is why there is an "i" extra in the word voglio. It comes from "volere" and normal grammar rules would make ï want" "volo", but that means "flight".