@arch It looks like whoever did this wanted only one p-trap but clearly didn’t install it properly. Also since there was no external venting available, they used an internal air admittance valve which is not a problem but it does require you to '''''check it's functionality occasionally''''' as that could also be a culprit (if everything seemed to work well before).
Example of how it works:[br]
[image|2362650]
[br]
Photo provided courtesy of [link|https://garynsmith.net/venting-101-air-admittance-valve-pros-cons/|this site]
Example of this valve is here: https://www.amazon.ca/gp/product/B0069KD28C/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Lastly, you know as everyone has told you, that slope is WAY wrong. Here is a sample photo of what it "should" look like:
[image|2362649]