If you created a new User under a company account, using the same email address as another account the employee already had in place, then yes they have two DocuSign Users under two DocuSign accounts and this can be a problem. The problem we experience if the email address is the same is any envelope sent to that email address gets randomly routed to one of the two accounts. Therefore the User has to check both accounts to find the envelope if they perhaps delete the email notification, so it becomes a bit of a hassle. The User can request closure of the personal account via DocuSign Support case.
I think there are variables in the merging of Accounts and again this request would require a DocuSign Support case. I have been successful only once on merging an account from a personal into a corporate account but the majority of the time I find out how many envelopes were created under the personal account and just have the User download the completed documents and close the personal account. So best option would be to create a DocuSign Support case, provide the User information and the two accounts to ask if a merge is possible.