Let’s start off with some examples:
Exhibit A: Hey, Jamey, did you get that e-mail I sent you this morning?
Exhibit B: Hey Jamey, did you get that e-mail I sent you this morning??
Exhibit C: Myra had the baby!
Exhibit D: Myra had the baby!!
They’re subtle differences, to be sure, but I contend to you today, people of the court of punctuation, that the insinuations of these sentences are completely altered when the second question mark or exclamation point is added. Let’s rewrite each of the two phrases, substituting the insinuated phrase for the second punctuation mark:
Exhibit B: Hey Jamey, did you get that e-mail I sent you this morning? Why on earth have you not replied to it yet? What kind of inconsiderate imbecile wouldn’t reply to my e-mail? You will rue this day. good sir. Oh, you will rue it.
Exhibit D: Myra had the baby! Listen to me when I talk to you! Look me in the eye and listen to the words coming out of my mouth, just like the baby that just emerged from Myra’s uterus! This is happening right in front of you and you don’t seem to care at all!
See the difference? Adding that second quotation mark makes a world of difference, as does the second exclamation point.
So please, if you’re deciding between one or two punctuation marks, please be aware of the actual message the recipient will receive. Use double punctuation as sparingly as possible.