I've double-posted when a thread is so active that by the time I'm responding to one response, other responses have been posted. In some of those instances, I've simply posted what I was writing, and then double-posted (and sometimes not even, because another response came thru in-between my responses), but make sure I quote what I'm responding to.
It's also harder to go back and quote multiple responses using a cell phone, so double-posting is less cumbersome, although more obnoxious.
In all cases, I try to limit it to rare circumstances.