Genesis 30:18

And Leah said, The Lord hath given me my reward, for that I gave my handmaid to my husband; even so shall his children receive a good reward, because they will occupy themselves with the law. And she called his name Issakar.

יששכר, a double reward, one for handing over the dudaim; the other, that I gave my handmaid to my husband to have children with.

ותאמר, seeing that she had stopped bearing children and had given her maidservant to Yaakov in her stead, she thought that she had been rewarded for this by G’d (for two reasons; 1) for helping her husband to father more children so that his indulging in sexual intercourse with a woman who could not provide him with children would not be accounted as mere physical gratification. 2) for having done her maidservant a great favour to become able to mother the children of such a righteous man as Yaakov. For both of these reasons she expressed the hope that G’d would reward her. When she subsequently indeed bore children for Yaakov again herself, she considered this as her reward from G’d and thanked Him for it. She made certain that her son’s name reflected her feeling of gratitude by naming him יששכר.

יששכר, this name (word) has two successive letters ש, one in order to express Leah’s gratitude to G–d for having been paid her reward, שכר, for having given Rachel her son’s mandrakes in order to help her to conceive more easily, and the second for allowing her an extra night with her husband, i.e. hiring him, שכר, so as to become pregnant from him again. (verse 16), [Normally, this arrangement is viewed with distaste by the Torah; but the Torah reported the whole sequence in order to show the reader that Leah’s intentions had not been to satisfy her libido, but to become the mother of another one of the twelve tribes making up the Jewish nation. Ed.] This is also the reason why when reading the Torah in public, the second letter ש is not read, i.e. cannot be heard by the audience. An alternate approach: the reason this letter in the name of יששכר here is not read aloud was that it was “stolen:” from Yissochor’s son יוב which was really ישוב, as we know from Numbers 46,24. When the latter had first been called יוב, he protested against this name so that the letter ש was added to his name by his father.