The question wasn't "should I apply to more than one school" the question was, assuming I am applying to one school, is that a positive or negative in that school's eyes. I'm well aware it's not the best strategy for getting accepted and attending.
I asked a pre-law advisor that exact question about a year ago. He said he was almost certain it would have zero impact, but if it had an impact, it would be a negative --- it would lead them to believe you were a) arrogant, or b) not serious, or c) both.
Granted, if you have numbers that are way better than that school's averages, a) wouldn't be that much of a problem (unless they perceived you that you thought you were an easy in). Option b) would require a more involved explanation, maybe as a separate statement.
To be honest, I don't know how they would know unless you told them -- and I don't mean by simply leaving a "What other schools are you applying to?" question blank on the application (lots of people just do that anyway for privacy), but specifically sending them a letter or something explicitly telling them. It seems like a non-issue unless you make/made it one.