Nobody wants to hear that the problem is a spreadsheet. It usually is. Before a single line of copy matters, the list has to answer boring questions: what is one record, what makes two records the same record, and what happens when a field is empty.
What that looks like in practice
Deciding whether the business or the human is the primary record — for B2B it's almost always the business, with a decision-maker attached as optional. Keeping name fields for actual humans instead of stuffing a company name into them. Catching the shared phone numbers and franchise main lines that will silently merge two real companies into one contact on import.
A partial record is still worth having. A business with a name and a city is a lead you can enrich later. A record you can't trust is worse than no record, because someone will act on it.