I am getting the error "Multiple rows in singleton select" when i run
my stored procedure.

The stored procedure has a select statement
Select Field from Table where Value= InputValue into OutputValue.

Now i know what the error means. If the field has more than one
record where it is equal to the input value, it will give you that
error.

However, looking at the data, i know for sure there is only 1 record
for each value of this field.

In fact, I copied the table, and had the SP select from the copied
one. If the copied table had no records, no error. If it had just one
record, i get the error.

What might be the cause of the error?