Комментарий к Берешит 30:18

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.

ותקרא שמו יששכר, “she named him Issachar.” Actually, she should have called him שכר, “reward.” Since she felt she had been rewarded for two actions, the handing over of her maid servant, as well as the handing over of her son’s jasmines, she indicated this by the second letter ש in the word שכר calling this son יששכר.