This is something that I experience on all new Power/AIX systems that I install:
System migrated to AIX 5.3 (or later) might experience double boot
When booting AIX Version 5.3 (or later) on a system that has previously been running an earlier release of AIX, you may notice that the system automatically reboots and restarts the boot process. This is how the firmware processes changed information in the boot image. This reboot also occurs if the process is reversed. A system previously running AIX 5.3 (or later) that is booting a release of AIX prior to 5.3 goes through the same process. This ″double boot″ occurs only once; if the stored value does not change, then the second boot does not occur. If you install AIX 5.3 (or later) and continue to use only that version, this double boot occurs once, and it occurs only if your system was running a pre-AIX 5.3 release before you boot AIX 5.3 (or later). Systems that are preinstalled with AIX 5.3 (or later) and use only that version do not experience the ″double boot.″
Some will panic when this happens, but it’s not an issue. It only happens once, on the first installation of the first AIX or VIOS LPAR you install. After that you will never see it again.