When i try to overwrite an amended existing pdf file via export I get
>> file already exists. Do you want to replace it?
>> ;'Cannot save to the file, because it is already open.'
This occurs even after Reader closed down.
Only way I could resolve is by closing down and reopening Indesign.
This happens when the reader is open in the background.
If you are using windows check with Task manager > Process
if Mac than check Activity manager.
End the reader process from these places and see.