User merges occur when two users are rolled up into a single user. Often this occurs because several users share the same user credentials. A common example of this occurs when the user credential is set to the user's first name. Merges can also occur when an anonymous user provides credentials.
The best way to prevent user merges is to make sure that the user credential is unique to each user. For example, we recommend using email address as a unique user credential if your app has users log in via email and only one user can be associated with a given email address.