I think this excerpt of a letter (published in the Hosafot of Likutei Sichot volume 30, Parshat Vayeitzei) from the Lubavitcher Rebbe, written Rosh Chodesh Tammuz 1981, may be addressing this issue.
If I understand it correctly, The Rebbe is responding to someone who wrote to him about something the Rebbe said about Yissachar vs Yisshasschar.
The Rebbe clarifies that he wasn't discussing that halachic ramifications, or how we read it in the Torah. Those issues are discussed in other places. Rather he was pointing out the inherent contradiction in the way people pronounce the name of Seferim (everybody pronounces both letters) and the way people pronounce the name (pronouncing only one letter).
It's a little hard for me to be 100% certain that this is what the Rebbe is writing, since I don't have any context (maybe someone can clarify it), but it seems from here that the Rebbe is saying that people pronounce the sefer with both "sins".