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It is possible to label multipoints such that the centroid of the set of points for a multipoint is labeled.  The function gs:Centroid is used the SLD.  The centroids are calculated on the fly and can be symbolized using the same attributes as the original points.

Boundless wrote up a detailed explanation of this solution.

In this example of points it can be seen that if a multipoint has only one point, the centroid is on top.  If the multipoint has two points, the centroid is in between the two points.