SAS creates the variables in the order it sees them. In your case 3 versions of the variable Key are created 2 from the original tables and 1 from the coalesce. The select statement determines the order, so in your case the key that is used is comming from the table Main. If you don't want to change the names try altering the order ie.
select coalesce (Main.Key, Minor.Key) as Key, *
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