I can find a lot of apps that will do it for me automatically... But they all seem to suffer from the same set of problems.
If they can automatically identify dupes they don't really look at the data. For example, I keep ending up with contacts that have two addresses that are really the same (Like "123 Main St, Somewhere PA 12345 USA" and "123 Main St, Somewhere PA 12345"). The main Google contacts merge does this...
Or they aren't smart about identifying duplicates (I have a lot of "sparsely" populated contacts that have the name "Info" but are info@companyA.com and info@companyB.com and so on). A lot of apps do this for companies because I don't have a person's name in the First and Last fields.
Or lastly there are plenty of apps that will link contacts but not actually merge them (why bother?)
I'm willing to put in the work I just want to select one or more contacts, pick the data I want to keep and merge away.