We know that functions are mathematical entities that assign unique outputs to given inputs. In other words, a function can't have repeated values in the input repeated x-value.
For example consider the relation {(1,4), (1,5), (2,6), (3,7), (4,8), (5,9)}.
We see that above relation has repeated x-value "1" which is assigned to two different output values 4 and 5. So by definition of function, above relation can't be a function.
