Why am I getting "Duplicate entry 'master-<username>' for key 'UK_RU8TT6T700S9V50BU18WS5HA6'" error?

Issue

I run RH-SSO on Openshift and I found "Duplicate entry 'master-' for key 'UK_RU8TT6T700S9V50BU18WS5HA6'" error is logged.
What is the issue?

Example log:

[0m[0m04:30:33,009 INFO  [org.keycloak.services] (ServerService Thread Pool -- 55) KC-SERVICES0006: Importing users from '/opt/eap/standalone/configuration/keycloak-add-user.json'
[0m[33m04:30:33,173 WARN  [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (ServerService Thread Pool -- 55) SQL Error: 1062, SQLState: 23000
[0m[31m04:30:33,173 ERROR [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (ServerService Thread Pool -- 55) Duplicate entry 'master-<username>' for key 'UK_RU8TT6T700S9V50BU18WS5HA6'
[0m[31m04:30:33,184 ERROR [org.keycloak.services] (ServerService Thread Pool -- 55) KC-SERVICES0010: Failed to add user '<username>' to realm 'master': user with username exists