I recently found a double trap under one sink (with a corrugated drain pipe thrown in for good measure) and a missing trap under another sink in the same house.
The double trap drastically slowed the drainage from the sink. The sink that was missing the trap had that nasty sewer odor smell coming from it.
P-traps (or older S traps & drum traps) actually have a purpose under the sink or tub/shower or in a toilet.
That U shaped bend under the sink traps a plug of water in it that keeps nasty sewer gases from coming back up the pipe into the house. Of course, it can also catch rings, earrings and other items that may be inadvertently dropped down the sink.
Two traps can allow a vacuum to form slowing or even completly stopping water from draining down the pipe.
It shouldn’t take the plumber too long to move the extra trap to the right sink.
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