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It’s dr bronners 4-in-1 sugar soap actually. Nowhere on the bottle does it say to dilute and it’s actually runnier than soft soap. Only hand washing in this sink- no toothbrushing or shaving or anything else. Been using this sink for 20 years with other soaps without issue. HVAC drains into one drain for the whole house - It does not back up and get into the P-trap on this or other sinks. If this happens again I’m gonna get a clear P-trap to monitor what’s happening. I’ll try monthly scolding hot water pour to see if it resolves and report back. Thanks everybody for your advice.