Important Note: The combination of Kasra+Shadda is displayed ambiguously in some Arabic fonts. When Kasra is combined with Shadda, Kasra is displayed above the base line, just below the Shadda, which might cause it to be mistaken by Fatha. For example, in the following word
There is a Shadda+Kasra above Reh, while there is a Shadda+Fatha above Yeh. The distinguishing difference is that in the combination of Shadda+Fatha, Fatha is put above Shadda, while in Shadda+Kasra, Kasra is below Shadda. The same issue applies to the combination of Kasratan (tanween) + Shadda.