It’s possible for a correspondence to be filed in 2 or 3 different files but only one file is available.
A good cross reference system is needed .Its a form of a card or folder directing a person to where a document which can be filed under more than one is found.
E.g. A letter Akola-Okubu might be filed under Akola-okuba file or in Okubu-Akola file
Where a letter from Akola-Okubu is filed under Okubu-Akola file a cross reference card is put under Akol-Okubu file to direct where its located’
In big firms where a photocopying machine is available, several copies are made and filed in all relevant files reach bearing the information on where the original is.