I think the concept is formally identical to database normalisation but applied to code. If the code is duplicated and you know the different part will have to

I think the concept is formally identical to database normalisation but applied to code. If the code is duplicated and you know the different part will have to evolve the same way: factorize now. If the different part are just formally similar by accident wait a little before factorize. Similarly in a database you create a table for users but you don't create a table for "family names" and "first names" even if you could factorize them : when Cassius Clay become Mohammed Ali you don't want to rename all Cassius to Mohammed and all Clay to Ali.