What this error mean " duplicate entry 'root@localhost0' for key '<group_key>' " ?

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SQL

hi , i was learning some SQL-injection , i type this in mysql console :

mysql> select count(*),concat((select user()),floor(rand()*2)) as a from informa
tion_schema.tables group by a

;
and after i enter the same query twice i face this error msg :

ERROR 1062 (23000): Duplicate entry 'root@localhost0' for key '<group_key>'

can you tell me why i face this error ?

What I have tried:

i search in google, but i dont understand .