When a container field have an OnObjectValidate script trigger attached, an Insert File script step which target that container will be performed two times.
This occur as long as a script step is performed by a button with a single action or with perform script option activated.
This not occur when performing Insert File menu command.
The only workaround is to disable the OnObjectValidate script trigger.
See attached sample file.
Configuration tested :
FileMaker Pro 14.0.2 and 13.0v5
OS X 10.10.5 and OS X 10.9.5