We would like to search for duplicate contacts/users in our Database outside of the currently available Email, First Name, Last Name, Marketo Unique Name/Code in the Duplicate Fields filter. We use quite a number of custom fields in our Database, and have discovered many cases of duplicate users where they all have the same general information, but a differing email address, NPI number, and so on. Is this feature currently possible in Marketo?
3 choices of vastly differing risk & expense:
A fourth option (which might be an extension of item 3 in @SanfordWhiteman's response) is to export all the data (including Marketo IDs), do your merge analysis external to Marketo (in a nice DB for example), and come up with a list of 'master' and 'duplicate' records, and then fire this data at the Merge API (it is easy enough to write a data loader using something like Python (and the very usable libraries that are available for it), or the programming language of your choice.
I just did something of this nature a few days back for a client, so can testify that is is comparative straightforward (but not without risk).