please look at records 25 and 29 - there is the same core number.
So I am assigning different CoresID with the same Cores Number - only 1 is required.
Because i can add to related table Cores ID = 25. I do not have add Core ID = 25 and Core ID = 29.
use a find duplicates query to find the duplicates and delete one or more of them.
Struggling to understand what you are doing since the related link in the other table will be orphaned - so those need to be removed first
with regards your second question - it depends on the purpose but certainly I would expect to see a unique index because you wouldn't want duplicates. Whether you also have a primary key is another question
