It is true that we suck at estimating. Doubling helps you get closer since you always think you'll be less busy and have less distractions. You can also get really good estimates by breaking all tasks down into things that are less than one day. However, this takes time as you are really doing investigation that is part of the task itself to figure out how many sub-day tasks you need for a bigger task.

In the end, estimating is bad for how people use it (imo). It is used to make gantt charts and plan release dates which are useless. It is much better as a personal tool to track how effective you are at turning concepts into working products. If you track time, it should be so you can reflect and improve.

We all really need to move to prioritized iterative development (not scrum) where the team is working on the highest value items in prioritized order. This also means cross training so people don't say things like "Jill doesn't know the image processing code so pick so other (lower priority) task". If you are always completing the most valuable tasks, everything else will come out in the wash.

In my experience, the bigger the project, the bigger the multiplication factor. On a big enough scale, multiplication by 2 might be not enough. Most often I find that implementation times can be estimated quite reasonably, but time for testing, documentation, team-coordination and communication are not factored in properly.

Then increase the unit of Time by one.

2 hours = 4 days

3 days = 6 weeks

2 weeks = 4 months

If you have already done it multiply by the Golden Ratio Φ. If it's a new task multiply by Pi π.

Multiply by 5.