@olivia_rust I had actually looked at that tool but for a different purpose so I had forgotten about it. My thoughts (related to merging, but not necessarily the current merge tool):

• It's not clear what the result of a merge will be. Does it keep all information from both? Does it remove duplicate information? Is any information lost?
  • It would be useful to have an option to decide which information to retain from each contact. This would have to be selected for each individual contact
• I have some contacts that should be merged but that do not match.
  • What are the matching criteria?
  • It would be useful to be able to select multiple contacts and be able to merge those.
• Is there any way to keep my Exchange and Google contacts in sync?
  Thanks
  Regards
  Keith Collyer

If a duplicate contact is found, you will get a list which will show all the contacts that were found and then you can decide per each which to remove, which to keep, which to merge etc.
The matching criteria currently is the Name. I have checked with our devs and it does not check based on the email only at this time, so the feature request for that would make sense.

Exchange and Google would each sync the contacts with the server, they cannot sync with each other.

@olivia_rust Thanks. I don't think you have answered my questions:

• It's not clear what the result of a merge will be. Does it keep all information from both? Does it remove duplicate information? Is any information lost?
  • It would be useful to have an option to decide which information to retain from each contact. This would have to be selected for each individual contact
    By this, I mean that for each potential duplicate, I should be able to decide which information should be kept. For example, say an address has changed for one contact, how do I know which address will be in the merged contact? As the deduplicate window is modal, I cannot check the contact information.
    It would also be useful to be able to manually select contacts and merge them. This does not seem to be possible at present.
    It seems I actually have three separate requests - would it be better to separate them?
    Regards
    Keith Collyer

@olivia_rust @keith It merges the data together. Since it looks for Name first, the name is the same, and then the additional contacts info (emails, phones...) is ADDED as additional contact info.
I recommend you perhaps create a test contact and try he feature yourself if you are worried about a specific field.

You can drag&drop a contact atop of another contact to merge it manually.