This Question/Answer seems close to mine, but I think I have a different problem.
Here's my query:
SELECT PL.PlantName AS 'Plant Name', M.MachineNumber AS 'Line', P.Type AS 'Record Type', P.Reason, P.ActivityDate, P.OrderNumber, P.Bundle, P.Quantity, P.ItemLength, P.Pattern, P.PartOption, P.InvCoil, P.Footage, P.Material, P.ScrapCode, P.EmployeeName, P.LBperFt, P.DelayCode, P.DelayReason, P.Duration, P.CostPerLb, P.ListId, P.ListText FROM dbo.Production P WITH (NOLOCK) JOIN dbo.Plants PL WITH (NOLOCK) ON P.PartnerId = PL.PartnerId JOIN dbo.Machines M WITH (NOLOCK) ON PL.Id = M.Machine_Maintenance_Plant_Maintenance WHERE ActivityDate >= '2018-04-01' AND ActivityDate <= '2018-04-07' ORDER BY 'Plant Name';
Before I added the third table, the query was fine. After joining the third table, I'm getting 6 copies of each row returned.