If you have duplicate records, you end up merging the records. What happens to the documents that are present on these records.
We have two Contacts, one with Word “My doc” attachment and other with Word “Test” attachment.
When we merge the two records and select C 1 as master record. The record c2 will be set as Inactive and the master record will have two document locations.
That is how merge works for Documents. Hope this was helpful!