I am working on a contact cleanup project involving several years of customer and vendor records. The total dataset contains thousands of contacts collected from different systems and exports. While merging everything sounds straightforward, I'm worried about duplicate entries, overwritten information, missing contact fields, and formatting issues. Some records even belong to the same person but use different names or email addresses. For anyone who has managed a large contact merge project, what were the biggest challenges you faced? Were there any mistakes that caused problems later and should be avoided from the beginning?