I do mountain biking on some of my local trails and I have started doing some tabletop jumps which I can now clear, but I am afraid of starting to try doubles. Any advice?
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Such fear is completely normal. Eventually you'll get over it, and even though it's different for different people there are some things you can try to help you cope with that first time. Here are some approaches I've either used myself or seen with others - in the end it mostly only comes down to what CardMechanic already answered: in case you can properly jump tabletops, literally the only thing holding you back from jumping similar doubles is a mental block.
edit as requested, some info from the comments: the reason why I'm mentioning finding a double similar in shape/size/length to a tabletop you know how to jump, is that you can clear both with the same body movements and hence don't have more risk of over- or undershooting it.
Overshooting a double or tabletop is the same: if you have some amount of experience and spot your mistake early enough when in the air you'll by all means want to avoid landing front wheel first since that might end up going over the bars. Instead actively force the rear wheel down - but also not too much. Even when completely overshooting and having to land on flat that has a good chance of not resulting in a crash. And it's easier than, while in the air and possible panicking a bit, trying to aim for a landing with both wheels at the same time. Actually you can train for this to some extent: take a jump you know well, then try to land a few inches further. Then some more, and so on, and take mental notes of how it feels and what happens and how you can counter it.
For undershooting it depends on how much you undershoot. If it's only a bit, e.g. hitting the top of the landing with the rear wheel (not ideal, but unless you're really short you'll just ride away) or the front wheel (dangerous, often results in an uncontrolled 'bounce') there's no difference. If you undershoot way too much though, a tabletop will save you but on a double you'd land straight into the gap. Which can end up pretty bad. But can be avoid by not trying to advance too fast, i.e. if you can jump X easily, don't attempt a double which is X * 2. Take small incremental steps, as for any practice out there.
Once you can clear a tabletop, you can clear the same size and space double. It's all about removing the mental block that says it's a different jump. It's the same jump, with a hollow middle. That's all that's different. Just get your basics down, make sure you clear it cleanly several times on the tabletop, get muscle memory going, then try it on a similar size jump without the top. Good luck!
Something else to look at, check out the landing ramp when you move on to doubles. Riding tabletops will make you complacent about not watching where and how your wheels sit when you land. You definitely don't want to case the landing(hit the lip of it) with either wheel. You also don't want to undershoot it and land in the middle. But also be careful about not overshooting the landing ramp, as landing on flat ground also sucks. A whole lot of how the physics of jumping a bike work is based on landing on a decline.