Making a backup is the first and most important step when merging records. After you merge records, a duplicate record is deleted from ACS, so having a recent backup is the only way to reverse any changes made when merging records.
Once you've backed up your data, choose your Preferred and Duplicate records in ACS.
Ideally, the preferred record contains more information and is more up-to-date than the duplicate record. The merge often uses information from the preferred record rather than the duplicate record, and for this reason, it's important to look at both records in View/Edit Individual and compare the data before performing a merge.
When you merge records, the duplicate record is merged into the preferred record. Single-value dates, fields, and lists in the duplicate record are overwritten unless the same field in the Preferred record is blank. To see which data changes during a merge, see Merging Data in ACS People and Family and Contributions Data Changes when Merging Records. You could also print the duplicate record's People Profile report before you merge, so that you have a copy of the original information.
What if I need to merge more than two records?
You can only merge two records in ACS; however, if an individual has two duplicate records and one preferred record, you can perform two merges. First, carefully select your preferred record and your two duplicate records (the preferred record usually contains more information). Then, merge the duplicate record with the least information into the duplicate record with more information. Once you have one duplicate record, you can then merge the data into the preferred record.
In addition, you can edit your People Setup Settings to check for duplicates when entering new records. This can help you avoid merging duplicate records.