A question on logic. Why is that second if needed? I often see the "if and only if"(iff) structure in logic. however I am never able to figure out how it is dif

A question on logic. Why is that second if needed? I often see the "if and only if"(iff) structure in logic. however I am never able to figure out how it is different than a single if. what exactly is the difference between the following three statements. a theory should be considered scientific if, and only if, it is falsifiable. a theory should be considered scientific if it is falsifiable a theory should be considered scientific only if it is falsifiable