arcgis desktop - Can labels for overlapping points be combined/merged into one label?

I have points representing sample locations. Often, multiple samples will be taken in the same location: multiple points with the same location but different sample IDs and other attributes. I'd like to label all points which are co-located with a single label, with stacked text listing all the sample IDs of all the points in that spot.

Is this possible in ArcGIS using either the regular labeling engine or Maplex? I know I could work around this by creating a new layer with all the sample IDs for each location in one attribute value but I'd like to avoid creating new data just for labeling.

Basically I want to go from this:

To this (for the topmost point):

Without doing any manual editing of the labels.