Replace duplicate spaces with a single space in T-SQL

This would work:

declare @test varchar(100)
set @test = 'this   is  a    test'

while charindex('  ',@test  ) > 0
begin
   set @test = replace(@test, '  ', ' ')
end

select @test

Even tidier:

select string = replace(replace(replace(' select   single       spaces',' ','<>'),'><',''),'<>',' ')

Output:

select single spaces