According to past records, Apple introduced and released the 15" unibody redesign in Oct 2008. And then they introduced the 13" version in June 2009. That's about 8 months apart.
It's been four years, and my memory is not so good.
However, I've searched some old news published here and I found the press info released by Apple on this:
http://www.apple.com/pr/library/2008/10/14New-MacBook-Family-Redefines-Notebook-Design.html
MacRumors also published some news on this:
https://www.macrumors.com/2008/10/14/apple-announces-new-aluminum-macbooks/
In October 14, 2008, Apple refreshed its MacBook line, and released the following models:
-A white polycarbonate 13-inch MacBook, with a Core 2 Duo 2 GHz processor, for US$ 999;
- Two all-new aluminium unibody 13-inch MacBooks, with Core 2 Duo processors, clocked at 2 GHz and 2.4 GHz, for US$ 1,299 and US$ 1,599, respectively;
- Two all-new aluminium unibody 15-inch MacBook Pros, with Core 2 Duo processors, clocked at 2.4 GHz and 2.53 GHz, for US$ 1,999 and US$ 2,499, respectively;
- A 17-inch MacBook Pro (aluminium, but not unibody) with a Core 2 Duo processor clocked at 2.5 GHz, for US$ 2,799.
Apple also updated its line of MacBook Airs at the time.
At the time, all 13-inch laptops were called MacBooks (there was no 13-inch Pro). In the next refresh, the 13-inch unibody models became known as MacBook Pros to differentiate them from the 13-inch polycarbonate models. So, there was one (and only one) generation of unibody MacBooks.
So, the 13-inch unibody MacBook Pro was indeed released in June 2009, but just because Apple changed the name of the 13-inch unibody MacBook changed to MacBook Pro. It was, in fact, the second coming of the 13-inch unibody design. Look here:
http://www.apple.com/pr/library/200...for-Up-to-40-Percent-Longer-Battery-Life.html
As we can see, Apple redesigned the whole line MacBooks/MacBook Pros, except for the lower-end 13-inch model (because it would probably be too expensive to build it in aluminium) and for the 17-inch model. Apple would launch a 17-inch aluminium unibody in January 6, 2009, less than 3 months later (
https://www.macrumors.com/2009/01/06/apple-releases-aluminum-unibody-17-macbook-pro/).
As a result, only 3 months after the release of the unibody design, the whole family of MacBooks (13, 15 and 17 inches) had a unibody design.