As a point of general netiquette, it's considered impolite to double post because in some communities people use the practice to squelch other posters. A double post makes it more likely, for instance, that a new page in the thread will be generated, and since many people only read the most recent page, it can sometimes be an effective way to drown out an opposing viewpoint. In addition, since each post is a new row in the database, it can cause some small degree of bloat in the tables.
Double-posting in your own thread is viewed as more acceptable, as is putting in a second post after sufficient time has passed that people have probably already seen the first and had the opportunity to comment but not availed themselves of it.
There is a very functional multi-quote feature here that will let you easily respond to multiple previous postings in a single post of your own.
Some forums even have a module that automatically merges double posts without moderator intervention.