removing duplicates and calculating average values

[Frédéric Parrenin <frederic parrenin univ-grenoble-alpes fr>]

Dear all,

I often encounter the following problem in my data analyses.

I have two columns: one is an abscissa in increasing order, the other is 
an ordinate.
The abscissas have duplicates.
So I would like to remove the duplicates in the abscissas and to 
calculate the average value of the ordinates when there are duplicates.

How can I do that easily?

Thanks in advance for the help,

Frédéric