Duplicate parent/child data - sql-server

Question

I have some parent/child data across two tables. I need to copy the parent rows back into the parent table, but then also copy the child rows as child rows of the new rows created.
I have been searching this site and Google, but can only find examples from Oracle or that use XML (or have many warnings about not being reliable), so am posting here for a complete easy-to-refer-back-to solution.
Take the following code (SqlFiddle):
DECLARE #tbl_person TABLE
(
ID int IDENTITY(1,1),
person nvarchar(20)
);
DECLARE #tbl_drinks TABLE
(
ID int IDENTITY(1,1),
personID int,
drink nvarchar(20)
);
DECLARE #i int;
INSERT INTO #tbl_person (person) VALUES ('Bob');
SET #i = SCOPE_IDENTITY();
INSERT INTO #tbl_drinks (personID, drink) VALUES (#i, 'Beer');
INSERT INTO #tbl_person (person) VALUES ('Wendy');
SET #i = SCOPE_IDENTITY();
INSERT INTO #tbl_drinks (personID, drink) VALUES (#i, 'Champage');
INSERT INTO #tbl_drinks (personID, drink) VALUES (#i, 'Water');
INSERT INTO #tbl_person (person) VALUES ('Mike');
SET #i = SCOPE_IDENTITY();
INSERT INTO #tbl_drinks (personID, drink) VALUES (#i, 'Beer');
INSERT INTO #tbl_drinks (personID, drink) VALUES (#i, 'Lemonade');
SELECT * FROM #tbl_person;
SELECT * FROM #tbl_drinks;
This produces this output:
ID person
----------- --------------------
1 Bob
2 Wendy
3 Mike
ID personID drink
----------- ----------- --------------------
1 1 Beer
2 2 Champage
3 2 Water
4 3 Beer
5 3 Lemonade
I know how to easily duplicate a single person plus their drinks, but not multiple people. Assuming I need to duplicate Bob and Wendy I need to get to this output:
ID person
----------- --------------------
1 Bob
2 Wendy
3 Mike
4 Bob
5 Wendy
ID personID drink
----------- ----------- --------------------
1 1 Beer
2 2 Champage
3 2 Water
4 3 Beer
5 3 Lemonade
6 4 Beer
7 5 Champagne
8 5 Water
I cannot figure out how to compare the old and new parent ID columns in order to get the child data.

Answer 1

The problem is that INSERT doesn't really have a "from table" that you could reference in the OUTPUT clause. But you could achieve the same with MERGE statement:
declare #tbl_IDmap table (newID int, oldID int)
merge #tbl_person as target
using (
select ID, person from #tbl_person where ID in (1,2)
) as source(ID, person)
on 1=0
when not matched then
insert (person) values(person)
output inserted.ID, source.ID into #tbl_IDmap;
And then duplicate the drinks with the new IDs:
insert into #tbl_drinks(personID, drink)
select m.newID, d.drink
from #tbl_drinks d
inner join #tbl_IDmap m
on m.oldID = d.personID
Here is your SqlFiddle updated.

Answer 2

Determined to add some additional solutions (and having thought about this most of the night!) I am posting an additional solution that doesn't use MERGE, hopefully to help users with older versions of SQL. It's more verbose than #TomT's suggestion but works okay.
SQLFiddle
-- Gather the people we need to copy
DECLARE #tbl_IdsToCopy TABLE
(
[counter] int IDENTITY(1,1),
[existingId] int
);
INSERT INTO #tbl_IdsToCopy (existingId) VALUES (1),(2); -- Bob & Wendy
-- Table to save new person ID's
DECLARE #tbl_newIds TABLE
(
[counter] int IDENTITY(1,1),
[newId] int
);
-- Create new people and save their new Id's
INSERT INTO #tbl_person
(
person
)
OUTPUT
INSERTED.ID
INTO
#tbl_newIds
(
[newId]
)
SELECT
p.person
FROM
#tbl_person p INNER JOIN
#tbl_IdsToCopy c ON c.existingId = p.ID
ORDER BY
c.[counter]; -- use counter to preserve ordering
-- map the old ID's to the new ID's and find the drinks for the old ID's
INSERT INTO #tbl_drinks
(
personID,
drink
)
SELECT
n.[newId],
d.drink
FROM
#tbl_IdsToCopy c INNER JOIN
#tbl_newIds n ON c.[counter] = n.[counter] INNER JOIN -- <-- map the old person ID to the new person Id
#tbl_drinks d ON d.personID = c.existingId; -- <-- find the drinks of the old person Id
-- Results
SELECT
p.ID,
p.person,
d.ID,
d.drink
FROM
#tbl_person p INNER JOIN
#tbl_drinks d ON d.personID = p.ID;