To drive your point across, all one needs to do is make a practice of checking the alphabetical listing for every tree before viewing the tree itself. If one views that listing page by page, one can see the number of times a person's name, dob/dod, and location of event, has been entered. Once is enough for any person. The rest is trash and will need to be consolidated or deleted to ensure one complete profile for each person, with no orphaned doppelgangers dangling throughout the tree.
The slightest variation in a name, its spelling, or a relationship to another person is guaranteed to post as a duplicate person. If one ignores the rule, there will be much extra work required to restore the tree to health. There is now a Merge feature, which can facilitate making corrections, but one must make judgment calls in identifying the preferred or corrected entries for each person. That is definitely a learning experience!
Discipline is necessary to learn to enter a person but once in a tree and to link that one person to any event affecting their life. But, once learned, the tree owner may some day have a well-developed tree to display with pride.