Contention based and contention free preambles can not be common in real scenario. So there is no chance of collision.
As per this scenario, there can be below possibility
1. If both ue sending eNb selected preamble at same time and eNb is not able to detect any preamble .then eNb will not send rar.
- Suppose eNb is able to decode ratch preamble then it will send the rar with c-rnti. For correct ue , this will be the rach complete and it will not send the msg3.
But wrong ue should try for msg3 he should not get msg4.