“לישוב משפחת הישבי”
Rashi explains that ישוב was really יוב who was one of the seventy people that went down to Mitzrayim. The דעת זקנים in פרשת ויצא explains that his father יששכר said that יוב was not a proper name. He therefore took a ש from his own name and added it to יוב thus his name became ישוב.
In the times of the ראשונים there were two חכמי ישראל that were well versed in נקודות וטעמים. Their names were בן אשר and בן נפתלי. One famous machlokes they had was how to pronounce the name יששכר. בן נפתלי held that you must pronounce both שינין. However בן אשר held that only one שין is pronounced. להלכה there are many different opinions and minhagim.
The אבן עזרא ורד”ק both hold that that according to כללי הדקדוד only one ש is pronounced. The sefer דבק טוב adds that this goes well with what we mentioned earlier that יששכר gave one ש to his son יוב. The חות יאיר says the reason is because יששכר must divide the שכר of his תורה with זבלון. The minhag of Belz is that ישכר is pronounced and spelled with only one ש. This is also the opinion of Rashi in דברי הימים and most poskim.
The בית אפרים writes that some have the minhag to pronounce it with two שינין only until Parshas Pinchas. From there onwards it is only pronounced with one שין. Because it is in this parsha that ישוב is mentioned and it was then that he took one שין from יששכר.
The בני יששכר and the מנחת אלעזר were both נוהג that every יששכר in the torah is read with two שינין. Even though we usually pasken like בן אשר, when it comes to the name יששכר we pasken like .בן נפתלי
A fourth minhag is that of the חתם סופר, that only the first time in the Torah it is read with two שינין. The reason is to let everyone know that this name was chosen כי שכר שכרתיך בדודאי בני. Afterwards it is only pronounced with one שין since it is more appropriate not to publicize the reason.
The Steipler zt”l would instruct the בעל קורא to read it ישכר and then repeat it and say יששכר.
The נודע ביהודה (אבן העזר שאלה ק”ז) discusses how the name יששכר should be written in a גט, and how he is called up to the Torah. The question was about a גט where the name of the father of the מגרש was ישכר, and was written with only one שין. The נודע ביהודה writes that in Poland since everyone signs their name with only one שין and everyone is called up to the torah with only one שין pronounced, if it is written with one שין it is כשר. This is true even though in תנ”ך it is always written with two.
He continues that this is all true in Poland. However in the countries where he lived many people are called up to the torah with two שינין. Therefore if the father of the מגרש would be called up to the Torah and sign his name with two שינין and in the גט it is written with only one שין, then that is a שינוי השם and the גט is פסול.1
Prepared by R’ Avrohom Yehoshua Ziskind
1Sources:אוצר פלאות התורה,קריאת שמו”ת והלכותיה,נודע ביהודה