Just a hunch, but I don't believe Barrie's doing any copying.
More than once, I've been working on an answer, only to notice the StackExchange message: "1 new answer has been posted; click here to load." Sometimes this newly-posted answer is very similar to mine. At that point, I have a choice: I can either abandon my answer, or I can post it anyway. I've done both; usually, I'll keep my answer if I feel that it adds something to the whole conversation.
I've noticed that Barrie's answers are usually concise; I think he values brevity. My theory is that he's not trying to garner easy points by plagiarizing someone else's work, but that he'll post a brief answer if he think it does a better job of answering the question.1
Because his answers are short, it may seem like he's copying off someone else's work, particularly if his answer shows up 15 or 20 minutes after another. But looks can be deceiving. We don't know how much time may elapse between when an answer is started, and when it is submitted. I know that I usually proofread, spellcheck, and verify my answers before I post them, and touch up the formatting as well – all this can take time. For example, I noticed that Barrie used two sets of smart quotes in one on his answersone on his answers; perhaps he was double-checking dictionaries and changing 'quotes' to ‘quotes’ when your answer popped up.
1By saying "better," I don't necessarily mean that he thinks his answer is superior to the other. He may think it's better synergistically, that is, that the two answers combined – one brief, the other more detailed – may be better together than if one of them stood alone. Maybe he feels like he's supporting the earlier answer by providing independent concurring research.