Well, if you do it as per the above, you won't be overdosing.
Check your manufacturers instructions for correct dose - a little over that is not going to do any harm.
For example, Seachem Prime can be dosed at 5 times regular dose to alleviate nitrite poisoning 'in an emergency'. There is also a proper dose for regular water conditioning, and one for dealing with ammonia in tap water, to detox. So the question of what is an overdose depends on what you are using it for.
But anyway, dosing the whole tank prior to refilling is not an overdose, and in general a small overdose is not going to cause you a problem anyway.
Check your manufacturers instructions for correct dose - a little over that is not going to do any harm.
For example, Seachem Prime can be dosed at 5 times regular dose to alleviate nitrite poisoning 'in an emergency'. There is also a proper dose for regular water conditioning, and one for dealing with ammonia in tap water, to detox. So the question of what is an overdose depends on what you are using it for.
But anyway, dosing the whole tank prior to refilling is not an overdose, and in general a small overdose is not going to cause you a problem anyway.