Double posting is posting one after another, not necessarily the same thing.
Like, if you posted "Hi there everyone."
And then put another post, directly after, saying "I just ate lunch."
It's much less hassle to put it all in one post, rather than two, and also poor etiquette to double post.
If you need to add more information into a previous post, there's a wee edit button beside the quote button you can use.
And if you need to quote more than one person at a time, the little plus will allow you to MultiQuote.