Don't Want To Insert Same Values 2 Times.

create proc dbo.usp_CatchAddressException @tableName varchar(500) as
-- alter proc dbo.usp_CatchAddressException @tableName varchar(500) as


begin
set nocount on
declare @sql varchar(700)

-- truncate table tempException
set @sql = 'insert hava_dcde.dbo.STREET_EXCEPTION (id_town, id_voter, pre_value, nbr_exception, date_stamp)
select a.id_town, a.id_voter, substring(a.ad_str1, 1, len(a.ad_str1) - charindex('' '', reverse(a.ad_str1))), ''20'', getdate()
from HAVA_DCDE.dbo.' + quoteName(@tableName)+ ' a
where substring(a.ad_str1, 1, len(a.ad_str1) - charindex('' '', reverse(a.ad_str1))+1) not in
(select nm_street from HAVA_DCDE.dbo.state_streets ss)'
-- and a.id_town = + @tableName
-- print(@sql)

exec(@sql)
set nocount off
end
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when i run this SP 2 times, it will insert the values 2 times.
what should i do if i don't want to insert the same values 2 times?
I'm confussed, Should i use update? any input will be appreciated.