Duplicates are not determined by the number of people who use related sites (or follow other proposals). Duplicates are determined by the scope of the subject and the example questions being asked.
There's a blog post covering how we divide and scope sites:
That blog post has become a bit dated. We've evolved and refined our criteria for what constitutes a "duplicate site." The basic premise is "if the community is already being well-served by an existing site, the proposal is likely to be considered a duplicate." In other words: "does the proposal add anything new to the network?" We don't generally want to create vanity sites where folks want their own site splintered off just because.
But those are broad generalities that don't comprehensively describe all situations. It's preferable to discuss duplicates based on the their individual circumstances and merits.