What looks like “two flashes” is usually an E-TTL pre-flash plus the actual exposure flash. The pre-flash happens before the exposure for metering; only the second burst lights the photo. If you want to avoid that pre-flash, use the Speedlite in manual flash mode instead of E-TTL.

On Canon bodies, rear-curtain sync works with compatible Canon Speedlites that communicate with the camera, typically when mounted on the hot shoe or connected via a Canon-compatible cord/system. Some Canon bodies also offer a custom function for 2nd-curtain sync.

With non-communicating triggers, the camera does not send a special “fire at shutter close” signal. In that case, second-curtain sync generally won’t work unless the trigger itself can add a delay to simulate it. Some higher-end PocketWizard models can do that, but a basic manual trigger cannot.

So in short:

• the first flash is usually metering pre-flash, not part of the exposure
• switch to manual flash to eliminate pre-flash
• rear-curtain sync requires camera/flash communication, or a trigger that can add its own delay