[GlennS]

It covers up bad queries, so you may not see an underlying data duplication problem.

Often better to group explicitly so you know what's actually going on.

[matwood]

> It covers up bad queries,

Bingo. I used to work with a guy who would see duplicate results and just throw a distinct on his query. I had to keep on him to fix his queries or explain why distinct was correct in this case. My default is that distinct is almost always not the solution.