Confused by object pronouns
It seems that the object noun phrase is represented twice in some sentences, such as the following:
"I always write to my Portuguese friends." Translation: Yo siempre les escribo a mis amigas portuguesas.
In this sentence, "les" and "mis amigas" have the same referent. But as far as I can tell, not all Spanish verbs require this doubling of the object. How can I tell when I need to do this?