When closing a question in the iOS app as duplicate, the UI is very challenging. I understand this is not exactly an easy UI to do given the limitations, but still, it's very complex.
One of the worst experiences, though, is when you're voting to close a question that's already been voted for as duplicate by someone else's vote. You have to get the duplicate's URL and copy it, and then paste it - there's no automatic link. When I'm in the app, I usually find cases where I see an obvious duplicate from comments - I'm not doing tons of research, I only vote where it is obvious.
When there's a close vote as duplicate already pending, please give either a list of already chosen URLs, or something similar. Even if it's only possible when there's only one selection - this would be far better than zero, where you then have to go back to the post, copy the link from comments, and vote again.