When making registry changes, it is a good practice to export the key first. But many times, we are adding new items and not just modifying values on existing items. So if you import from the file you exported to, it does not remove the new items that were added. To do that, add a line at the top of the registry file with a minus in front of the key, like this, for example:
[-HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones]
This will cause this key, all it's subkeys, and items to be removed first, then the rest of the information will be added back. If the key does not exist, then no error occurs. This is probably a good practice to follow when putting together a .REG file to deploy so that nothing else will be under that key.