Why is there an extra 'Shin' in Yisachar?
Rashbam: It denotes a double reward, one for giving Rachel the Duda'im 1 , the other, for offering her maidservant to Yakov.
This is unlike those who explain that Leah was punished for her purchase (refer to 34:1:1:1***).
Why does Leah attribute her son to giving her Shifchah? Rachel did so, and did not have a son [yet]!
Riva: Rachel did so for she had no sons. Leah already had sons.
Why do we pronounce only one Sin in Yisachar?
Riva, Da'as Zekenim, Hadar Zekenim #1: One Sin is because she bought the right to lie with Yakov, and this is dishonorable. 1
Riva, Da'as Zekenim, Hadar Zekenim #2: Yisachar had a son Yov (Bereishis 46:13), and he thought that it is not a proper name, so he gave to him a Sin from his name and called him "Yashuv". 2
The other Sin is due to giving her Shifchah (Rashbam, Moshav Zekenim). We pronounce only that one.
Rashi (Bamidbar 26:24) says that Yashuv is Yov.
