Duplicate Check will do a proposal on what fields to keep, based on the merge rules. It will always pick a field with value over a field with no value. Manually decide which field value you would like to keep in the merge. The highlighted fields are the ones that will be kept in the merge.
You can change the proposed values on any field and write down your own custom value. The custom field will be highlighted in blue after editing it and its value will be kept in the merge.
Once you have selected the fields you would like to keep in the merge, press the 'merge' button. Duplicate Check will now merge the records into one quality record.