Why iMac (late 2012) does a double boot up every time?

Hi,

I have a 27inch late 2012 imac.I have a 3TB fusion drive running 10.8.2.

Every time I restart or start up from off, the machine does a double boot up.

The iMac boots up and the dong sound plays, apple logo showing.

Then 23 seconds into boot, screen goes black and switches on again,

Dong sounds plays again while the apple logo reappears.

The imac logs into desktop only after the dong sound and the apple logo appear twice.

any idea why would that happen?

Also, from time to time I get a small window popping up, but it dissappears a second later and I have no time to see whats wrong. I can only sort of make out that there is a yellow triangle on it. Probably some sort of error resolving itself?

Thanks!