SQL Server 2022

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

DECLARE @Demo AS TABLE 
(
  col bigint,
  expected bigint
);

INSERT INTO @Demo  (col, expected) VALUES
(521521, 521), -- repeat after 3 digits
(919191, 91), -- repeat after 2 digits, 3 times
(919291, 919291), -- no repeat - Note the 92 in the middle
(7676, 76), -- repeat after 2 digits
(123456, 123456), -- no repeat
(2147421474, 21474), -- repeat after 5 digits 
(2147483647, 2147483647), -- no repeat
(12341234, 1234), -- repeat after 4 digits
(123123123,123), -- repeat after 3 digits, 3 times
(123456123,123456123); -- no repeat

SELECT *,
  Corrected = CASE
    WHEN col / 100    % 100    = col % 100    AND col / 10000       IN (0, col % 100   ) THEN col % 100
    WHEN col / 1000   % 1000   = col % 1000   AND col / 1000000     IN (0, col % 1000  ) THEN col % 1000
    WHEN col / 10000  % 10000  = col % 10000  AND col / 100000000   IN (0, col % 10000 ) THEN col % 10000
    WHEN col / 100000 % 100000 = col % 100000 AND col / CAST(10000000000 AS bigint) IN (0, col % 100000) THEN col % 100000
    ELSE col
  END,
  col / 100000    % 100000,
  col % 100000,
  col / cast(10000000000 as bigint)
FROM @Demo d

col	expected	Corrected	(No column name)	(No column name)
521521	521	521	5	21521
919191	91	91	9	19191
919291	919291	919291	9	19291
7676	76	76	0	7676
123456	123456	123456	1	23456
2147421474	21474	21474	21474	21474
2147483647	2147483647	2147483647	21474	83647
12341234	1234	1234	123	41234
123123123	123	123	1231	23123
123456123	123456123	123456123	1234	56123