【排水逆流】原因はダブルトラップ？

It's been over two years since the bathtub in that house was completed. Whenever I empty the tub, water always backs up from the drain... I did everything from dismantling the bathroom to re-building the concrete floor myself, so I thought it was caused by a plumbing error. I learned for the first time from a comment on a video I posted about my impressions after using it for six months that the double-layered drain trap was causing this problem. The amount of water backing up wasn't that much, so I left it alone, but I've been getting comments on this video lately, so I decided to make a video about it.