When two UE's send same preamble to eNodeB , eNodeB responds to the preamble and sends message-2 in downlink. Now both the UE's receive the same message-2 in which parameters for msg-3 transmission in Uplink will be same. So both the UE's tries to send the msg-3 on same UL resources given in msg-2.
Now eNodeB cannot decode the msg-3 due to overlap of msg-3's from both UE's. eNodeB will not send any msg-4. So both the UE's back off after contension resolution timer expires and resend the preamble again.
This time there is less probability to have both UE's to select same preamble again