Let’s say this is part of your family history:
Your cousin has discovered new information about Marge Bouvier:
She sends you her file. With GenMerge, you match her file to yours. GenMerge identifies two common individuals, and merges each into a single person:
Your merged family history file has new information, with no duplicates!
GenMerge: the fast, easy and safe way of merging genealogy files.
Source: Simpsons Wiki