Let's say a user logs in to a site with their Facebook account, then later decides to log in with their Google account. Should a separate account be created for each method of login or should the user be able to log in to the same account using various login methods? If the later, how would it be recognised that it's the same person logging is using a different method? Or, would the option to merge one account with another be better? Is there a best practice for this situation?
The expected behaviour would be logging into the same account. This only works, if you give the user the option to connect their social media SSO into their account.
If the user is not connected it would create a new account. You could ask the user something like "Do you have an existing account? Connect now" and give them the option to login into their already existing account and the option creating a new account like "or create a new account here".
In my opinion, Bungie's Destiny 2 connect is a good example for that. You can connect to steam, xbox, psn etc. with one account or create multiple (even though not recommended)