I'm someone who does that I believe. I don't see it as a bad thing. I'm not going to go and pretend it was my idea, it's just that I take a while to make up my idea of what's right, and I'll discuss edge cases and all kind of issues with others really thoroughly and strongly, I'll do that with many people, about the same thing. And then I'll kind of form my own opinion from that, which could be 90% of person's X, and 10% of person's Y, or any other combo, sometimes mixed in with some of mine as well, but not always, it's very possible I was the most wrong of all initially.
In practice, it means that when I commit behind an idea, I'm often right. But the ideas are not necessarily mine. And I don't care.
I wouldn't say it's narcissistic, for me, I feel it's actually the opposite. I don't care that I came up with it or you or who did, or even if multiple people contributed to it, I just care that it's the right idea, and once I'm confident that it is, I will strongly upheld it and hold it as my opinion. That's because I've already put it to the test by arguing against it in all possible ways, talking to many others about it, getting their opinions, their alternative ideas, etc.
It's not a bad thing if people are careful about giving credits. I have seen both extremes of that:
1) The type who does exactly what the OP said, treat people like trash and think when they commit to an idea, they are often right. When they change their opinion, they don't give credit either, since they want to maintain the image that they are often right. I have seen great teammates left due to those people. I would not want to work with those people either.
2) The second type also hold strong opinions. When they change their view, they attribute it to people who brought it up. They don't care about maintaining an image that they are "often right". They genuinely thanks the people who brought them good arguments that changes their strongly held opinions. It's not about who invented the idea. But they understand it's good to appreciate people from which you get the new idea. I love working with those people. I try to give proper credits to people when they show me new ideas.
And I don't think the worst part is failing to give credit necessarily. People legitimately forget things, and they independently come up with the same ideas. The problem is when it's wrapped in egotism and inability to see that the person who suggested it earlier did so with good intentions, and might be worth listening to in past, present and future.
In practice, it means that when I commit behind an idea, I'm often right. But the ideas are not necessarily mine. And I don't care.
I wouldn't say it's narcissistic, for me, I feel it's actually the opposite. I don't care that I came up with it or you or who did, or even if multiple people contributed to it, I just care that it's the right idea, and once I'm confident that it is, I will strongly upheld it and hold it as my opinion. That's because I've already put it to the test by arguing against it in all possible ways, talking to many others about it, getting their opinions, their alternative ideas, etc.