I've seen a lot of misinformation on the forums regarding the t10 4pc bonus recently and wanted to take a moment to clear a few things up.
1. The extra slam proc CANNOT be overwritten by another Bloodsurge proc unless it too is an extra slam proc.
2. The extra slam proc lasts 10 seconds as apposed to just 5 seconds like the regular Bloodsurge proc.
3. The scrolling battle text will say [Extra Slam] when you get an extra slam proc. You can also monitor it by using addons like QuartzProc.
4. The shortened GCD from using the first slam of an extra slam proc applies to all abilities that are not currently on cooldown, not just the next proc'd slam.