Merging one customer account into another will:
add any existing credits/subscriptions from the old account to the new one
leave any credits or subscriptions that already exist in the new account untouched
remove the old account from existence in your CRM (this is permanent)
To do this,
copy the new account's email (the one you'd like to keep)
then open the old account
paste the new email into the field where old one is currently listed and Save
#duplicate customers
#duplicate emails