Sometimes, a source is so awesome, you want to refer to it multiple times in your document. ACS follows this rule:
"Numerical reference citations are numbered consecutively from the beginning of a paper. When, occasionally, a reference is repeated in the text, the original number is used, a new number is not given."
For example, the number 23 can come after 42 in your in-text citation. The easiest way to do subsequent citaitons is to manually add the endnote as a subscript number.

But at the end of the paper, the citations are all in numerical order in the references page.
