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This is a picture of a garbage disposal under a kitchen sink, whoever hooked this drain line up used two traps instead of just tying into to the trap that was already there. Every plumbing fixture requires a trap
Hooking up two traps like this could cause a siphon that will cause the water to be sucked out of both traps, allowing sewer gases to go into the home. A real plumbing contractor would have never hooked the garbage disposal this way.