Merge data within the same categories first before working on cross-type dups. For example: Client vs client, Partner vs Partner; Prospect vs Prospect, etc. It helps to narrow down the batch size and is probably safer when starting to merge dups. (save me a lot of headaches) Building Data Quality/Merging score: depends on your Salesforce and fields. It would help when merging cross-type dups (i.e Client vs Prospect). If you use the mass merge tool, make sure to analyze the data and data type first. Then adjust the merging rule behavior for each field if possible. For example, the tool I'm using allows me to concatenate the long text field if this field is populated in all dups. Or get the sum amount or max number depending on the fields.