A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Multiple punctuation marks - Omission of punctuation marks
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Part III. Style 20 spelling

The guidelines given throughout this chapter sometimes call for the use of two punctuation marks together—for example, a period and a closing parenthesis. The guidelines below govern the omission of one of the marks and the order of the marks when both are used.

21.11.1 Omission of punctuation marks

Except for ellipses, never use two periods together, as when a period in an abbreviation ends a sentence. Keep the abbreviation period when a sentence ends with a question mark or an exclamation point.

The exchange occurred at 5:30 p.m.

Could anyone match the productivity of Rogers Inc.?

If a situation calls for both a comma and a stronger punctuation mark, such as a question mark or a dash, omit the comma.

If he had read “What Do Dogs Think?” he would have known the answer.

While the senator couldn't endorse the proposal—and he certainly had doubts about it—he didn't condemn it.