Ask, before publishing, whether a page would be a new version of one you have.

Body {"title": "...", "client": "...", "client_path": "..."} (only title is needed). Answers {"match": null} or {"match": {...}} in the likely_revision_of shape, plus slots_left (free pages left, null on a plan that does not count them). Creates nothing: a client named here that has no space yet stays without one. When there is a match, PUT /sites/{match.id} updates that page in place (same link, no page slot) instead of publishing a second one. Owner-authed.