For or(x,x) with identical inputs the output is the same as the input. Putting it into a statement: it will rain or it will rain. Its the same as only dealing with just it will rain and whether thats true or not.
For or(x,y) when x and y are both true or both false they have the same output as or (x,x). That makes sense. Since when both are true and when both are false they have the same input.
So or (x,y) is the same as or(x,x), or or(y,y), when the x and y represent the same thing.