Duplicate Results in View with OG Relationship
Okay, so I believe I traced the issue to a problem with Entity Reference Relationships in Views, and posted a bug. 

However, I was able to solve the duplicate issue AND get the results I wanted (all titles from related groups) by taking a different approach in the View. 

Instead of hook_query_alter() or hook_views_pre_render(), I just wound up adding the field that reference the groups, setting the Formatter to "Label", and checking the "Link label to referenced entity." This produced a comma-separated list of groups linked to the group page (exactly what I wanted).