Originally Posted by Entrekin
Some cameras (usually compacts but some dslr models) will due a preflash, which in some cases, is like have a strobe light go off several times quickly before a final big flash. This is most likely due to the person using an Automatic mode. You wouldn't have much control of that. If you meant just a regular one time flash, it is something you would have to get use to. Some photographers prefer flashes just for extra light or use a set up (stands, umbrellas, etc) to get enough light for a look they want. So it would all depend on if you were talking about a hallway shot or actual shoot.
I think what you are talking about is a feature to reduce or eliminate red-eye. That is a real pita for cosplayers because pretty much everyone will assume the photo is taken after the first flash goes off (though the ones that do more of a strobe are a little better). I remember having that issue with my first DSLR. I was trying to take a photo of my host family for a holiday (in auto mode) and the SLR in question just did two flash instead of a strobe. 9 times out of 10 the kid in the group would stop posing and either do something goofy or just look away.
Preflash is an un-discernible first flash of light that happens in i-TTL/p-TTL/etc so the camera can gauge exposure.