EnodeB does not identify an UE based on IMSI, it means eNodeB can't identify new connection request is same as already existing UE connection.
If MME receives attach request with an IMSI for which it already has UE context then first it check whether this request is from genuine UE or not. For this it performs authentication and security procedure. If authentication/security gets passed then MME proceeds with Create Session Procedure . If SGW already has context then it will delete the context locally and proceed with Create Session Procedure.
If you have specification reference which states "MME sends reject in this case", please share.