D. Set up a staging database, and define external IDs to merge, clean duplicate data, and load into Salesforce.

This approach involves setting up an intermediary staging area (database) where data from both legacy systems can be consolidated. In this staging database, the data can be reviewed, cleaned, and deduplicated effectively before being loaded into Salesforce. External IDs can be used during this process to help with the identification of unique records and to assist in the deduplication process. This ensures that when the data is finally migrated to Salesforce, it is of high quality, free of duplicates, and structured correctly for Salesforce's data model.