Canon SPEEDLITE EL-1 Advanced User's Manual page 59

Note
The second-curtain sync works well when the camera's shooting mode is set to
< > (bulb shooting).
When the flash mode is set to < >, the flash fires twice. The first flash is a
preflash to determine the flash output. It is not a malfunction.
To return to normal flash, select < > (first curtain sync) in step 3 (< > is
not displayed on the screen after setting).
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