javascript - Why does the jquery selector ("id,id") selects all elements with duplicate IDs -

i came across year old code written developer (yes, knew him personally) access elements having same id.

$("#choice,#choice") 

it returns elements having id. if use below

$("#choice") 

it returns first match, expected.

after searching time, i'm unable figure out official links pointing technique, how selected elements duplicate id.

can please explain how working ?

update

please see question not alternative use. i'm aware of classselectors , attributeselectors , know having duplicate ids not recommended, have live years old code way (if know mean).

http://jsbin.com/zodeyexigo/1/edit?html,js,output

if @ code of sizzle.js jquery uses selecting elements based on selector understand why happens. uses following regex match simple id, tag or class selector:

// easily-parseable/retrievable id or tag or class selectors rquickexpr = /^(?:#([\w-]+)|(\w+)|\.([\w-]+))$/, 

but selector in question $("#id,#id") not match selector , uses queryselectorall (line no 270 in ref link), replaces selector "[id='" + nid + "'] " (line no 297 in ref link) selects elements matching id.

however, agree other people in thread, not idea have same id multiple elements.