Using a relationship in a view will create a view entry for each combination of original content to be listed and relationship.
For example, listing users with a relationship to all of their created content will list an entry for each node created by that user, even though the listed fields may only be user fields which will be identical every time.
To fix this, in the views edit advanced section, use the "Use aggregation" setting. Using all default values should already be enough.
Modules views_aggregator and views_merge_rows did not work properly and could cause errors which can break every single view on the website even with their default configurations. When they did work, they also caused wrong number of rows to be shown because they operate at page level instead of at the level of the database query. Do not use these.
Grouping by field using the display format settings also didn't work as that is purely visual grouping and doesn't remove or aggregate anything from the result.