A function must have one y-value for any given x-value.
Usually, if you see the same x-value in the x-column more than once, that's not going to be a function, because that usuall makes a single x-value pair with more than one y-value.
This would be NOT be a function, because x-value of 3 is paired with two different y-values.
x | y
3 7
4 8
3 9
6 7
Can you identify the function?