First of all, you have to know about the physical significance of correlation. that is if y=function(x) like equation ..means for each value of x there should be a value of y. but sometimes practically we get more values of y corresponding to each value of x. in that case number of y values should be identified.
for example
if x=distance and y=number of car covered the distance with velocities ....given in the table
in this case you can take mean (or mediun or other) of the velocities for each distance and then calculate correlation.
