This question already has an answer here:
What's the rule about sentences that end in something that already has a period in it?
Like a sentence that ends in: Washington D.C. -or- etc.
Personally, and I realise this is wrong, I would love to have two periods - the first being, of course, the mark for the abbreviation and the second being the period/full-stop of the sentence.
I'm guessing that it is just treated as a stone that kills two birds, but I've never heard a definite answer as to how to deal with these.