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I have a Sony NEX3 camera and an old external flash. Since Sony NEX3 doesn't have a hotshoe I use the optical sensor in the flash to be triggered by my camera's flash.
Unfortunately the camera uses two flashes (double flashes even when the red eye is off) so it triggers the off-camera flash before the shutter captures it. What can I do regarding this problem?
One good solution is to get a Hot Shoe Flash Adapter so you can use your old external flash right on the camera, or get a smaller Manual only flash to trigger your external optical slave flash.
$10 Amazon Hot Shoe Flash Adapter MSA-10 for Sony NEX 3
An even better solution is to get this Wireless radio transmitter/receiver:
$10 Wireless Flash Trigger Hot Shoe Adapter with Receiver for Sony NEX3
The first flash is probably for light metering (Canon call their version e-TTL, Nikon iTTL if you want to read about how it works), but the external flash doesn't know that of course.
You should be able to get an external optical trigger that can handle this (a hotshoe with a tripod thread on the bottom). Optical triggers are cheap so this would be a viable solution. Slave triggers that will do what you want apparently exist: "For example, I have a very-inexpensive underwater optical slave that can be set to count various numbers of pre-flash pulses before arming for the "real" flash" (photocentric.com).
Here's a list (dpanswers.com) of some models that apparently ignore a preflash, or attempt to smartly ignore a string of preflashes. The brand reviewed there is Wein (models HS-XL D, PN-XL D, L8 D and XL8 D -- might be out of date now).
Apparently the Seagull SYK-5 Trigger can be set to ignore red-eye reduction flashes -- it might work for you (Amazon in the hope that the link won't die too soon).
Some Yongnuo external flashes can be set to an optical slave mode (davidpartington.com) if you're shopping for a new flash.
If you're using a disconnected non-dedicated external flash like this, it'll be unmetered - you'll have to set all the exposure and flash parameters manually to get the image you want.
The first flash from your onboard flash is a metering flash for auto flash exposures. With manual exposures like this, it's neither needed nor even useful.
So, set your onboard flash to a manual exposure mode (and the rest of the camera, too). It'll only give a single flash, at the right time, which your external flash will pick up at the right time.
An alternative to the optical triggers or a new flash that Chriss H recomended.
If your external head allow it, lower the power to 1/2 - 1/4, so the flash has enough power to trigger twice.
The problem is that your camera can "think" there is enough light so it won't fire for the exposure.
You can not have all on life.
Try enabling red-eye prevention. That may sound silly, but for power saving reasons, some cameras will use the red-eye prevention preflash (if it is just one) as a metering flash so this gives the external flash and the phototrigger a quite larger grace period before the second flash. You'll still get two flashes of course.
Another option is using packaging tape (or ND filter foil) layers over the external trigger until the preflash no longer manages to fire the second flash. Of course that's more icky.
Dedicated flashes often have slave modes of themselves (my Metz flash with SCA3202 foot has one). Those are usually immune to anything but a proper "main flash": the other flashes (metering or redeye) are apparently "marked" in some camera specific way that dedicated slaves know to ignore. Or they may even be able to participate in the metering flash but TTL with multiple flashes is an icky subject.
