I made a comment on another thread Drain line configuration about the issue of counting trap arms. I started another thread to avoid thread drift.

A trap arm is the pipe between the trap weir and the vent pipe. Many codes allow only one trap per trap arm. There are sometimes installations, especially under kitchen sinks, where two drains, each with a trap, merge at a wye (or a tee) before entering the vertical pipe that is the vent. So, the trap arm is split. The question is whether this is one trap arm which is split, or it is two trap arms with a common section.

I have done some googling and have found a number of references, all consistent with calling this situation one trap arm.

Here is the most definitive:
ICC Bulletin Board: Multiple Traps on one Trap Arm

Here (look for the color highlights) is an exception for "residential kitchen sink installation." This is an example of "the exception proves the rule." If the only way one could have two traps on a single arm was to have the traps in series (double trapped), why would this exception exist?
7359 Ordinance

This poster's plumbing inspector would not allow two traps into a common line before the vent.
Install dishwasher on second trap?

Here is another example.
double sink: one trap or two traps? [Archive] - Terry Love's Plumbing & Remodel DIY advice forum

This inspection report seems to agree. See highlighted colors.
Status: Case Status

Thoughts?

- BOB

Interesting.....

I believe double trapping has the potential to cause future problems. In which case, I would highlight it to my customer.

When two traps are hooked up in series the potential issue is not siphoning but that the traps will tends to clog up more frequently.

When traps are in parallel you could have the potential for a venturi effect and water could be siphoned out of one of the traps under certain conditions. I would think that under most conditions that this is an unlikely scenario. However, with today's deeper sinks and two sinks draining at the same time, we could see a problem occur.

Jeff Euriech
Peoria Arizona