Looks like your PI sender channel is picking the file before even it is completed. That's why it is triggered partially for the 1st time and then as a full file during the 2nd attempt.
Try to set 'Msecs to Wait Before Modification Check' in the sender channel to some 5000. This will make the channel to fail if it tries to pick the file while writing and will wait till the file is completely written & available.
~Srini