I just don't get it... even after studying various grammar pages and asking my argentinean friends: Why is it: Ellos le leen una revista a ella. (They read her the magazine) It sounds to me like this sentence is 'too much*, because 'le' already refers to her, why 'a ella'? Literally translated it would be: They read her the magazine to her? Why you need to double the indirect object?
"le" is easier to write and say than "a ella". As soon as you established in the conversation it was a "her" and not a "him" you were talking about you can drop the "a ella". "A ella" is just for the clarification of "le". In the real world you will probably mostly see object pronouns like "le" used with no clarification. And since it is easier to say and write it, it probably is a de facto rule that you must use. "Ellos le leen una revista a ella" = "Ellos le leen una revista".
"le" is just a sign-post telling you that the verb will have an indirect object, i.e., a person "to" or "for whom" the action is directed.
"Le" can refer to "él", "ella", or "usted", so unless it's clear from the context who "le" is referring to, you have to add it in at the end. It does sound repetitive-- if you're going to have to say "a ella" anyway, why put the "le" in?-- but that´s just how it works. As Rocko said, once the context is established you only need the "le"