OS X Double Login

Something that happens on my iMac (which has 2 login accounts) is that when I log into the main account, I get asked for my password twice.

So the login window shows, I put in my password, its accepted & then (after a moment for the computer to think) I'm asked for the same password again with exactly the same login screen (although the login box is in a slightly different place on the screen).

Its a little annoying and I wonder if it is something that can be stopped?

 

the login screen is just the password entry box below my profile pic. Desktop behind is obscured

 

No not using File Vault. The other account is my housemate's but on the occasions I've seen him login, doesn't seem to be an issue, nor on my aging MBA (which is only my login). Assume it is to do with having 2 accounts on the iMac.

 

Indeed it has. Thank you. Apologies for the late response.