This is because I don't think the same thing being added multiple times is a huge issue. The same word can have multiple meanings, slightly altering meanings, or some people are more extensive in their description than others. I think that is a feature of a crowd-sourced dictionary - you get to see how people describe it instead of one curated description. At the end of the day, it's up to the user to see which description they find most compelling, helpful or useful.