Trigger - ORA-6512

SqlPlus - Sql Server 7.3.2.2.

                         I've the following trigger:
                         Create or Replace Trigger TBI_RA1
                         Before Insert on Rime_Alteracoes for each row
                         Declare
                         Cursor C_dummy is
                         Select max(num_alteracao)
                         From rime_alteracoes
                         Where cod_projecto = :new.cod_projecto and
                         regiao = :new.regiao and
                         ano = :new.ano;

                         Cursor C_Crs Is
                         Select distinct 1
                         From rime_alteracoes
                         Where cod_projecto = :new.cod_projecto and
                         regiao = :new.regiao and
                         ano = :new.ano and
                         num_crs = :new.num_crs;

                         dummy rime_alteracoes.num_alteracao%TYPE := 0;
                         BEGIN

                         Open C_Crs;
                         Fetch C_Crs into dummy;
                         Close C_Crs;

                         If dummy = 1 Then
                         raise_application_error(-20100,'Coluna única
                         repetida');

                         Else
                         Open c_dummy;
                         Fetch c_dummy into dummy;
                         Close C_dummy;

                         if dummy is null then
                         dummy := 0;
                         else
                         dummy := dummy + 1;
                         end if;



                         :new.num_alteracao := dummy;


                         :new.login := USER;


                         End if;
                         END;

                         When I try to Insert a line in the table
                         Rime_Alteracoes, it gives me
                         the following errors:

                         insert into rime_alteracoes (cod_projecto,
regiao,
                         ano, num_crs)
                         values (1,'N',1997,1)
                         *
                         ERRO na linha 1:
                         ORA-20100: Coluna única repetida
                         ORA-06512: na "RIME.TBI_RA1", linha 21
                         ORA-04088: erro durante a execução do trigger
                         'RIME.TBI_RA1'

                         The first one, I understand, it's the error
that
                         I've made and it should
                         appear, but the other tow I don't understand!?