Database error

[Operador del sistema]

please, help, i have this problem when a second user try to log into imp:
Database
error:
Invalid SQL:
insert into
active_sessions
( sid, name,
val, changed
) values
('f46fcea0626660d3c4e86afcc1579944',
'HordeSession',
'base64:JHRoaXMtPmluID0gJyc7ICR0aGlzLT5wdCA9IGFycmF5KCk7IA==',
'20010109122942')
MySQL
Error: 1062
(Duplicate
entry
'HordeSession'
for key 1)

that appers on the web browser...

thanks for help

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