I assumed that happened because there wasn't anything blocking water from flowing between the two (typical t-fittings for drains have a plastic piece to prevent this) So, I tore that out and made it look like this.
That's all well and good, but I have the same problem. I think it's because the drain in the wall is high compared to the bottom of the sink. I had someone suggest using a double wye at the wall; his thinking was the angles would prevent water from flowing back.
Is this legal? Are there better ideas?