When attempting to merge duplicate constituents together under Constituents > Duplicates > Merge two constituents, you notice that when combining two constituents who each have a membership that it no longer combines the memberships into one record. Additionally, the "Allow multiple memberships" option is not selected for the Membership program.
Ex. John Doe and Jonathan Doe are duplicate records who each have memberships on their record. You merge them together and there are now two separate memberships on that record.