How do I duplicate rows?

I have difficulties solving a problem. I'll try to explain the problem the best I can. 

I got two tables: Table A and Table B.  Due to some poor programming the tables are not normalized and this is what's causing my problem. 



Table A:
- Field A ID (Unique)
- TableB.FieldA (Foreign key)
- Field X
- Field Y
- Field z
etc
(Field A and Table B ID is a joint primary key)


Table B:
- Field A (Old unique key)
- Field B (Foreign key)
- Field C (new unique key)
(Field A and Field B  ID is a joint primary key)

Due to poor programming Field A and Field B were used as a joint primary key, but conceptually only Field A should be a primary key. Because of that the it sometimes happen that the same key in field A appears multiple times so I had to make a new primary key in Field C with unique keys.

Table A uses TableB.FieldA as a foreign key. Multiple rows in Table A can have the same Foreign Key. But because I had to make a new unique key in Table B (FieldC), those data in Table A with the same foreign key also belongs to the new keys made in TableB.FieldC. 

Example: 
[quote]
Table B

Field A   Field B   Field C
-------  -------  -------
1..........700.......9000
1..........701.......9001     (ID 1 appears 2 times in Field A. Not allowed)
2..........700.......9002
3..........700.......9003


Table A
Field A   Field B  (from TableB.FieldA)    Field C
-------  ----------------------------  ---------
1..........1........................................Some text
2..........1........................................Some text
3..........1........................................Some text
4..........2........................................Some text
5..........2........................................Some text
6..........3........................................Some text
7..........3........................................Some text
- 1 - 
[/quote]

As you see in the first quote ID 1 appears 3 times as Foreign key in TableA.FieldB. In Table B you see that ID 1 got two new ID's (9000 and 9001). That means that for every row in Table A with the old ID 1 I have to insert two new rows for ID 9000 and 9001. And that's my problem. How do I duplicate the rows in Table A based on the number of new ID's made in TableB.FieldC?

Don't be afraid ask question to clarify the problem if needed. It was difficult to write down the problem so I don't know if I got the problem well explained. I want the result to be like in the code-quote below:
[quote]
Table A
Field A   Field B  (from TableB.FieldA)    Field C
-------  ----------------------------  ---------
1..........1........................................Some text
2..........1........................................Some text
3..........1........................................Some text
[b]4..........9001...................................Some text
5..........9001...................................Some text
6..........9001...................................Some text[/b]
7..........2........................................Some text
8..........2........................................Some text
9..........3........................................Some text
10.........3........................................Some text

Rows in bold are new rows
[/quote]