Why can't ids used twice?

Question

i have used them twice. it is wotking properly

Answer 1

Better use Classes instead...

Answer 2

Who said that they can't? (it might have side-effects but it's valid)

Answer 3

ID is an idenfication for a particular element. It's just in its definition. Like in schools two kids don't have same ID. Two elements can't share the same ID.
On the other hand, classes is used to group elements. Elements belong to a class.

Answer 4

in this app they said it. check chapter 3.