#1. "RE: Double flash with R1C1 and mirror lock up" In response to Reply # 0
I assume you are using TTL flash control in which case that is to be expected and completely normal. When you lock up the mirror, the monitor pre-flash sequence fires and when the shutter opens, the triggering flash fires.
#2. "RE: Double flash with R1C1 and mirror lock up" In response to Reply # 0 Thu 16-May-13 12:27 AM by aolander
If you use manual flash, this wouldn't happen. You'd have to dial in your flash exposure with your aperture and power level. Also, I doubt that mirror lock up is going to make much of a difference when using flash as the flash exposure is so short. You get a pre-flash and an exposure flash when using TTL without mirror lock up , too, except that they occur nearly together so you don't notice it. The pre-flash doesn't use much power.