You seem a bit confused about IDs.
Any given ID must only occur ONCE in your document.
And any given element may have only ONE ID.
The ID gives the element a UNIQUE identifier.
It is not the same as CSS classes, which can be duplicated on different elements, and as well you may have more than one class on the same element.
Many of us prefer to minimize the use of IDs. They are over-rated! ;)