No, there should not ever be two traps on a single drain line.
The point of the trap is to create a water barrier which prevents sewer gasses from coming up the empty pipe and into the house.
When you have two traps on the same line, you end up creating a kind of vapor block in the line which prevents it from draining properly.
What do you do?
Get yourself some 1 1/2" PVC (not CPVC, which is yellow, PVC which is white) with a 90 deg elbow, and also get two Fernco couplings (black plastic couplings with straps on each end that can be tightened) that will fit the pvc and iron drain pipe.
Use a metal blade on a sawz-all and cut out the p-trap in your basement. (Have a bucket handy!) Leave room to connect the Fernco coupling on both sides. I'd cut the right hand side out just past the coupling.
Measure and cut the PVC so that with the 90 elbow the ends line up with the newly cut drain lines. Glue the PVC together and confirm the fit, then connect to the old drains with the Fernco.
Viola - a proper drain line that will drain smoothly w/ no vapor lock.
The typical mid-trap joint of a plastic P-trap does not have a washer. Instead there is a tapered end that fits down into a smooth beveled hole on the opposite piece. The tightening of the nut forces them together to form a tight seal. Your's looks to be this type.
If this joint is leaking for you then there are several probable causes.
- There is crud on one or the other side of the tapered part of the joint that keeps it from sealing 100% when the nut is tightened.
- The nut may have cracked and is not able to hold the necessary pressure to keep the tapered pieces together.
- One or the other of the pipe joint ends may have cracked due to aging or faulty plastic.
- The smooth tapered parts on one or both parts of the joint may have been cut or scratched.
It may be possible to stop the leak by a thorough cleaning of the parts. If it is a damaged part then the simplest fix may be to purchase a new set of trap pipe, nuts and washers and just replace the thing. These parts are not at all expensive.
Best Answer
You should be able to find a replacement pretty easily. I put "home depot p trap" into Google and came up with the following product in under a minute:
I believe this would work for your application. Where I am, they list it as available in-store, so you should have luck finding it in store. Otherwise, you could order it. The one in the image is specifically called "1-1/2 in. Plastic J-Bend."