I close to double mine, but it also depends on the tasks. Some tasks I just know I'll complete in 1X my estimate. I do have a rule that nothing is ever estimated at less than 2 hours; no matter how small the task. Invariably, any time I have broken that rule, someone has checked in code that won't build (or something similar) and I have to spend time dealing with that.

I notice some devs like to underestimate to make them look more productive. Most of those tend to spend a lot of time fixing bugs that QA pushes back to them. They sometimes spend ever more time arguing that it's not their bug.