Here's a different method that might work well. Should we start testing? 😀

DECLARE @code nvarchar(4000) = '12333345566689';

with seed1 (a)

as

(

select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all

select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all select 1 union all select 1

),

numbers (n) as

(

select top (LEN(@code)) row_number() over (order by (select null))

from seed1 s1, seed1 s2, seed1 s3

)

select a.b.value('(./text())[1]', 'nvarchar(4000)') as [text]

from (

select CASE WHEN SUBSTRING( @code, n, 1) = SUBSTRING( @code, n - 1, 1) THEN ''

ELSE SUBSTRING( @code, n, 1) END

FROM numbers

for xml path (''), type

)a(b)