In the past I made a big mistake to create structured keywords or hierarchies like they are called in LR. This has been a huge mess and time lost for me trying to fix the mess . Double keywords, you cannot flatten them, so you can never get rid of this hierarchy nightmare.
Lightroom does not have it, but I would jump in a heartbeat to PM Plus if there was an option to flatten and make disappear all the structured hierarchy inside the metadata of the image. Just one button, select images and flatten all the structured keywords. A dream to come true someday, I hope.