You will find these instructions (or some variation) on most shampoo bottles. Why include these instructions in the first place? Does it mean that the shampoo is actually not that good and you need a double dose? Or, could it possibly be they think you are extra dirty and need the extra round of cleaning (which might be kind of insulting)?
Actually, it is neither of those reasons.
The shampoo is not necessarily faulty, nor are you likely that gamey. The reason they put those instructions on the bottle is that they know the secret of doing a job effectively. The first round cleans most of the pollution, grease and dirt out of your hair - the next round does a final pass to make sure your hair is actually clean, and not just "less dirty".
Other two-part solutions abound - floss before you brush your teeth; your teeth will be healthier if you loosen up the stuff that is stuck between your teeth and then brush and rinse it away.
Painting a room? Don't ever believe the "one coat" label on the can or the brush. Every good painter (DIY or professional) knows that you need (at least) two coats for a good finish. One time round is simply not enough, unless you are freshening up the room with the same exact colour within a couple years. But if you change colours, you will need two coats, and possibly a primer/sealer before that to help hide the old colour. And the bigger the colour change between old and new, the more coats you are likely to need to cover it.
Not surprisingly, the same approach applies to Project Planning, however we are not limited to just a second pass; depending on the length of your project you may end up doing multiple rounds of re-planning to make sure that things are going to get done effectively.
Because - as every Project Manager knows, your project plan becomes obsolete the moment you save it or print it out.
Listen to the podcast, or read the full article on Gazza's Corner blog.