If wonders work anything like they do in Fallen Enchantress, then you cannot place a wonder in the build queue if another nation has already placed said wonder in the build queue, regardless of whether or not work has actually begun on it. If you want to build a wonder, you need to be the first to place it in the queue, which might also require that you are the first to research the tech unlocking the wonder.

This works both ways - the AIs cannot attempt to build a wonder if you have it in your build queue, even if you're only going to start building the wonder 200 turns from now, and you cannot attempt to build it if an AI has it in their build queue.