Blimey - no wonder she isn't engaged yet!
I would just completely stop talking to her about wedding stuff. If she tries to tell you what she likes, just change the subject. If she asks, just tell her it's a surprise. That way she can't complain that you've copied her. And even if she does want to feel like you've copied her, that's her issue.
If she's got nothing else troubling her at the moment, and is just being a pain about your wedding, I would try and distance yourself from her for a while - not even saying congratulations is out of order in my book, and honestly I probably wouldn't even invite her with behaviour like that, for fear she would try and make the day all about her!
But honestly, even if you have similar things, the feel of one wedding to another is completely different. If she gets her wedding in first, it's no big deal. Would I be annoyed if my friend deliberately chose the same dress as me - yes. But as long as she didn't wear it to my wedding, I'd get over it.
I'm sure people are aware of her behaviour, so try not to stress over it. Just take my initial advice, and don't talk weddings to her anymore!