It’s a warning that can appear in two situations:
- You are logged into Counterwallet elsewhere (as in, for example, another of the Counterwallet.io servers (at this moment hosted by the Counterparty project there’s three of them, cw01, cw02, cw03 (counterwallet.io))
- Potentially a bug in Counterwallet software
If you just logged out, wait 45 seconds and try again, but otherwise you can ignore the warning.
Is it possible that someone else is using my pass phrase?
Not unless they got it from you. Counterwallet never knows what your password is. Even when you create your wallet the suggestion you see in your browser is created on the client.