When I want to close a question as duplicate using the app, I have to enter the question ID as usual. Problem is, as far as I can tell, the question IDs are shown nowhere! Please add the ID somewhere.
The question ID is found in the URL for the question, the same way you would find it on a regular web browser. In my opinion it'd be too confusing to have "copy question id to clipboard" or something similar as a feature somewhere in the app because most the people who would find it would have no use for it.
For your scenario of sending duplicate votes you can either use the "Share" functionality to copy the question's URL to your clipboard but since it sounds like your phone for some reason does not give you that option you could also open it in a browser, copy the URL from there, then paste it into the flag dialog. The input field on that dialog accepts both inserting a regular question ID or inserting a URL.