I am definitely NOT the best person to do this catalogue. But at this point, no one else is doing it in a way that allows for ready identification of a specific area that might come up in a research project.

The ISOGG Wiki "Identical by descent" page's section on "Excess IBD sharing" comes closest, and DNAPainter uses that page's problem regions to warn about potential problems when you are doing chromosome painting in DNAPainter.)

Family History Fanatics also have a nice concise page on pileups which builds on Debbie Kennett's 22 Jan 2018 post "Small Segments and Pileups.

I would really like to include a web page about GEDmatch Q-Matching which addresses the SNP-poor regions, but I have yet to find a good web page on Q-Matching (although the Family History Fanatics page noted above does address it well though not completely). There is a good 2 Aug 2020 statement by Wilbon Davis in a reply a query on the GEDmatch forums that is worth quoting "Be sure to recognize that Q only helps eliminate segments that are accidental matches. Rely more heavily on cM lengths for estimating kinship and IBD filtering."

So I am doing what I can, with what knowledge I have.
-- Wesley Johnston

When examining DNA matches, some regions of shared DNA come from a shared common ancestor. These are called IBD (Inheritance by Descent). But others come from various other situations that are not indicative of the DNA coming from a recent common ancestor. These are in a group of match situations called IBS (Inheritance by State).

While there are more forms to consider, I will leave it at this being a catalogue of excess IBD of cases of too many DNA matches on the same region of the same chromosome. I had originally thought to include IBS due to a shared ancestral population (IBP), but those are not included.

I do think that it is very important for qualified researchers to research each of these and see just what they tell us. They are not useful for identifying common ancestors, and the catalogue is helpful for identifying when you have stumbled onto one of those cases, which is the main purpose of the catalogue. But I suspect that they have importance that we need to understand. This is fertile research ground for some enterprising well-qualified researcher seeking to "boldly go where no [researcher] has gone before".

At this point, I do not plan to classify any of these, as to which type they are or as to what they might mean. I simply want to capture all cases where this is found. I began (7 Feb 2021) a C22 Pileup Project, but I have not worked on it for a very long while now, and it really never got off the ground.