Scenario:
Calls come in on 200ICP (Atlanta) analog trunks, routed via speedcall to 3300 extensions (Boston) via IP trunks. Call is answered, then xfered BACK to 200ICP extension via IP trunks. If user on 200ICP is already on an outgoing local call (via analog trunks), new xferred call from 3300 barges in on 2nd appearance (multiline) and disconnects original call. This happens 2 out of three calls. Incoming local trunk calls to the extensions do not do this.

Is this a 200 or a 3300 issue?

If a person is on their ext in the SX200, and someone simply calls them from an ext. on the 3300, do you experience the same symptons?
Or, is it only happening in the scenario you described?

No. The only time it occurs is in the scenario described above.