That would mean every international flight would have to have its fuel measured and verified on arrival to work out what proportion was used in the EU. Even before a penny was raised in duty this would cost a great deal of money -plus of course the army of inpectors who would be needed to ensure compliance have to be paid. Who would pay -you guessed it, the airlines.
Once again, this has little to do with the environment and everything to do with treating our industry as a milk-cow.