The Hebrew for the tribe of Issachar has two shin letters. The first has a dagesh lene and hence doubles. However, the second one does not have its dot to distinguish sin from shin. What is the reason for this apparent omission?

The Ibn Ezra (Shmot, 1, 3) explains that just as in the word trumpeters, m’chatz’tzrim (which appears in Chronicles 6 times, e.g. Divrei HaYamim II, 10, 13) is pronounced m’chatzrim or machtzrim, because the 2 identical consecutive letter are difficult to say and are pronounced as one, so too, the second Sin in Yissachar is silent. Whenever a letter isn’t pronounced, it doesn’t have dots. It is written with 2 Sins to have the connotation: Yesh Sachar, “there is reward”.