If both duplicate contacts have the same Custom Fields – with data contained in one record which is different from the data contained in the record you are merging with – you will lose the custom field data in the record being merged into the ‘master’ record – as the master record will become the main record with that custom field information contained.
On the other hand – if the record you are merging into the master record has additional custom fields that the ‘master’ does not have – these additional custom fields will be pulled over as part of the merge.
Our advice is to always select the contact record with the most information and the most up to date custom field data and then merge any duplicates into this contact record.
To find out more about merging contact please click here to view a short video.