Duplicates gone before you ever see them.
Nothing is assembled before the target is described in writing. Everything removed is logged, so the removals can be inspected. Delivery includes a plain statement of what the file will not do.
Second-opinion pass
Having somebody who did not build the file check it against the brief.
Match confidence
Separating confident pairs from probable ones, and reporting the latter rather than folding them in silently.
File assembly
Records drawn together against those criteria, deduplicated, and checked for completeness.
Send a sample and we will tell you what we would change about it.