It really sucks to re-enter manually large number of people.
To avoid big merging operation its is possible normally in source systems like Gramps to select only one small part of tree and teh safely import it and join my merging possible only very few simple mergers.
Anouther approach could be to use advanced matching in inport and update only differencies.
In some system proeparing the familu data their is unique index for persons and families even.
If that qould have been adopted in orifinal import no problem.
But it is criminal to take away this time saving tool from PRO users, who normally really know what they have are doing.
Merging is pretty simple and quick if you know the person/profiles related.