Please help, I have work hard to make this easy to understand. (example below)

Currently, I am using a Vlookup formula to€¦ Search through product codes for
a certain code€¦ Then I have it return the transaction number€¦
So, the product codes repeats itself whenever is involved in a transaction.
However, transactions only occur once. (EXAMPLE BELOW)

My problem is€¦
When I use my Vlookup formula and drag it down, it will repeat the same
transaction for several rows (probably based of # of rows until the next
transaction with that product code) before listing the next transaction.

How can I eliminate this repetition process€¦SO, How can I create a Vlookup
that will€¦Search through product codes for a certain code, Return the
transaction number€¦THEN go on and only list the next transaction with that
product code.

Example- Searching for product code 21€¦

DATA SHEET
A-Product Code B-Transaction #
18 AX144
21 AT888
55 BT444
89 EE789
21 BW123
10 CR559

VLOOKUP FORMULA SHEET
A- =Vlookup(21, A1:B1000, 2, false)
AT888
AT888
BW123
BW123
BW123

WHAT I WOULD LIKE:
AT888
BW123