The most frequent number of samples of PMW data for an IMERG grid box in any given 30-minute period is overwhelmingly zero, with one in most of the rest. In a very few cases there are two or more. However, the histogram of single values of precipitation rate differs substantially from histograms of two, three, or four averaged together. Once the team realized this several IMERG versions ago, they instituted a down-select scheme to provide the single "best" PMW estimate in each grid box at each time. The GEO-IR brightness temperatures used are from the Climate Prediction Center (CPC) Merged 4-km Global IR data product, which gives the "best" GEO-IR value each half hour in each ~4x4 km (at the Equator) grid box.  The down-select of GEO-IR to a consistent number of samples (here, one) in a half hour is done at the CPC, which ensures that the GEO-IR values are all drawn from nearly the most consistent record achievable.  Inter-satellite differences and differences in retrievals due to surface climate regime are still realities for both PMW and GEO-IR.  

To summarize, there are multiple values for both PMW (occasionally) and GEO-IR (with a frequency depending on the satellite being used at any given time and place), but:

1. Organized records of the not-selected passive microwave samples do not exist - although a user could construct this from the PPS archive of all the Level 2 products.  
2. Accessing “all” the GEO-IR is a daunting task - and these data would be brightness temperatures, not precipitation estimates.

As such, IMERG is best described as "half-hourly snapshots".  The IMERG team suggests that the most sensible way to use these data is to treat them as the average for the half-hour period, since there is no additional information about the sub-half-hourly sequence of events, particularly in the case where the IMERG value is based on "morphed" passive microwave values from other times.