The IP Logins report provides an easy way for you to track duplicate logins in your instance. Note that your login report may be called something slightly different or located within a different menu item, depending on your instance's customization: 

 

 

Within this report, the 'Count' column reports how many different IP addresses were used within one day to access one user's account, so you can see when you may have more than one person using the same credentials to log into a single user account. (Note that IP addresses may change when accessing via mobile vs desktop so only 2 IP addresses in one day may not necessarily reflect duplicate logins.)