What you do about having two BSA ID#'s depends on how the person received their two numbers.
Easier to resolve is when a person has a second account created in error in the same council. Talk to your registrar to find out which number to keep. They will end the wrong number in the my.scouting data. You will need to contact Scoutbook support if the person’s Scoutbook record needs to be changed.
Things are handled differently when a person transfers between councils, because the old number can’t be used in the new council. The person receives a new number when their account is created in the new council. Use Manage Member ID under the My.Scouting.org menu to connect the old and new #'s. This should allow training recorded under the old number to show under the new number.
Getting the old records in Scoutbook to show the new number can take additional steps. If the new account in My.Scouting does not sync with the old account in Scoutbook (requires perfect match of first name, last name, and birthday), then a duplicate account is created for the person. Contact Scoutbook support to delete the duplicate account and update the ID# on the old account. If not done already, they can connect a scout account to the new unit.