The 4 Minhagim

Parshas Hashavua ויצא   There are 4 different Minhagim when to start pronouncing ישכר instead of יששכר. 1) The first time the name appears. (פ' ויצא). 2) From the second time & on (פ' ויגש) 3) From Parshas פנחס & on 4) From Parshas שמות & on Some, who claim, that the Minhagim 1-2-3 are based on reasons 1-2-3 1) According to Dikduk ישכר is the proper pronunciation therefore we always pronounce it correctly (even the first time) 2) The first time we pronounce the original name as it was given. From then on we cover up the 2nd reason. 3)  The changeover from יוב to ישוב is in פ' פנחס therefore we do the same with the changeover from יששכר to ישכר. 4) I don't know. Perhaps the עולם has a Pshat Reasons 1-2-3 to pronounce it ישכר. 1) In דקדוק there are times when two same letters next      to each other to pronounce only the first letter.     . מחצצרים דהי"א ט"ו:כ"ד is pronounced מחצרים    2) The two ש's in יששכר symbolizes 2 times שכר a) The שכר of giving זלפה to יעקב b) The שכר of Leah giving away the דודאים so she can spend an extra night with יעקב. Being the second שכר is a לשון גנאי (not appropriate to publicize) therefore we drop the second ש. 3) The second ש was given to יוב his son who in Parshas פנחס is called ישוב (see רש"י במדבר כ"ו:כ"ד) מפרשים write It was discovered later on of an עבודה זרה named יוב so they removed one ש from יששכר and added it to יוב and it became ישוב.