You asked "how bad". So to flesh out @RobertKoritnik's (entirely accurate) answer a bit...
That code is incorrect. Incorrect doesn't come in shades of grey. This code violates the standard and is therefore incorrect. It would fail validation checking, and it should.
That said, no browser currently on the market would complain about it, or have any problem with it at all. Browsers would be within their rights to complain about it, but none of the current versions of any of them currently do. Which doesn't mean future versions might not treat this code badly.
It also might select the first one, or possibly the last one, or select none of them, or crash the browser entirely. No way to tell without trying it.
"How bad" then depends entirely on how strictly a particular browser implements the HTML spec, and what it does when confronted with a violation of that spec.
What jQuery seems to be doing is selecting me the FIRST one, which in all cases is exactly the one I DON'T want.
I'm working around this by writing selectors to specify the region of the page I want to get my field in (ie: $('#containerDiv #specificElement') ). But there's one answer to your question--jQuery on Chrome definitely behaves a particular way when confronted with this spec violation.