After merging two databases together, you may find that the "same" person exists more than once. They're actually two different people to VSys One - each has its own ID code, addresses, assignments, etc. - but they refer to the same real-world person. The Find duplicate people tool can help you locate those duplicate people and the Possible duplicate people tool lets you verify duplicates and then combine them together.
Both of these tools are part of the Database merge tool and not available in VSys unless that tool is enabled.