Duplicated rows (x) amount of times in a table

You can get it using a table value or a value list, and a CROSS JOIN.

INSERT INTO foo
SELECT CONCAT(SO, t.x) as SO, SO_Line
FROM   foo
CROSS JOIN (VALUES('-1'),('-2'),('-3'),('-4'),('-5')) t(x)
WHERE  SO = 'ABC';
GO

15 rows affected

SELECT * FROM   foo;
GO

SO    | SO_Line
:---- | ------:
ABC   |       1
ABC   |       3
ABC   |       5
DEF   |       1
DEF   |       2
ABC-1 |       1
ABC-2 |       1
ABC-3 |       1
ABC-4 |       1
ABC-5 |       1
ABC-1 |       3
ABC-2 |       3
ABC-3 |       3
ABC-4 |       3
ABC-5 |       3
ABC-1 |       5
ABC-2 |       5
ABC-3 |       5
ABC-4 |       5
ABC-5 |       5

dbfiddle here

Instead of use a value list, you could use an INLINE User defined function like this:

CREATE FUNCTION tvValues(@Num int)
RETURNS table
AS
RETURN 
(
     SELECT TOP (@Num) ROW_NUMBER() OVER (ORDER BY S.[object_id]) [item]
     FROM    sys.all_objects S
)
GO

Then convert your query to:

INSERT INTO foo
SELECT CONCAT(SO, '-', t.item) as SO, SO_Line
FROM   foo
CROSS JOIN tvValues(7) t
WHERE  SO = 'ABC';
GO

NOTE: I've used tvValues(7) but you can use other values.

This is the result:

SELECT * FROM   foo;
GO

SO    | SO_Line
:---- | ------:
ABC   |       1
ABC   |       3
ABC   |       5
DEF   |       1
DEF   |       2
ABC-1 |       1
ABC-2 |       1
ABC-3 |       1
ABC-4 |       1
ABC-5 |       1
ABC-6 |       1
ABC-7 |       1
ABC-1 |       3
ABC-2 |       3
ABC-3 |       3
ABC-4 |       3
ABC-5 |       3
ABC-6 |       3
ABC-7 |       3
ABC-1 |       5
ABC-2 |       5
ABC-3 |       5
ABC-4 |       5
ABC-5 |       5
ABC-6 |       5
ABC-7 |       5

dbfiddle here

Create a table containing just a number column and insert the numbers 1 ... max, where max is at least 10. Then cross join this table with the result set

INSERT INTO Table (SO, SO_Line, ...)
SELECT SO + '-' + CONVERT (varchar, Numbers.number), SO_Line, ...
FROM
    Table
    CROSS JOIN Numbers
WHERE Table.SO = 'ABC' AND Numbers.number <= 10

CROSS JOIN creates all combinations from the rows of the first and the second result set. So if you you have 3 rows of Table and 10 of Numbers you will get 3 * 10 = 30 combined resulting rows.

Note, that his allows you to easily specify a variable number of repetitions (up to max) by just editing the number manually, through a parameter or from a column.