problems with Fks

I´am making foreign keys in my DB. Everything is working fine at first buy
now I´am having problems in the generation of this FK.
In some tables I wan`t to make some FKs to diferents tables.
But I can only make the first FK. Whe I wan´t to make the second I have this
error from IBExpert.
I have changed the orders to make the FK and i can make ever the first one,
but in the second I have the same error.
I have thist problem in some tables only. In others I can make the FK i
want.

I`am really lost with thist.

---------- STATEMENT ----------

alter table MOVIMIENTOS_CAJA_OPERADOR
add constraint FK_MOVIMIENTOS_CAJA_OPERADO
foreign key (MOVIMIENTO)
references MOVIMIENTO(CODIGO)


---------- ERROR MESSAGE ----------

This operation is not defined for system tables.
unsuccessful metadata update.
STORE RDB$INDICES failed.
attempt to store duplicate value (visible to active transactions) in unique
index "RDB$INDEX_5".