I don’t know why I didn’t Google this until today, but it’s always bothered me the default “Duplicate Spread” feature in InDesign tosses your new page to the end of your document—always forcing you to have to drag it elsewhere.
To duplicate a page inline with the page you’re actively working on, hold down your option key (Mac OS) and drag the page you want to duplicate into the position you want it duplicated to (usually right after the master you’re duplicating from).
It seems the default should be to duplicate in-line, in the context of where you are working in the document. I’m not sure of a use-case where I’d want a single page thrown to the end of the document, unless I’m duplicating a batch of pages into a new chapter. In that event, I’d rather the single page duplicate in-line by default and a multi-select of pages request a decision similar to the PDF insert pages dialog.