The stock version has the ability to send an email campaign based on a date field in the contact record. The most common use of this feature is to send an email to users on their birthday.
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We thought it necessary to take a new and fresh run at developing that feature in an actual addon that is not vulnerable to IEM updates and is compatible with all version 6.0+ and with the most popular existing addons on the market today.
Your current best option is to export the two (or more) contact lists and attempt to merge them into a single excel file. Sort by one of the columns and search for duplicates. It is tedious for certain but it also is a huge time waster.