Your Unique Tax Reference will usually stay with you for good once it has been assigned to you, as this identifies who you are on HMRCs records. There are however, some exceptions.
If you are made bankrupt, then a new UTR will be assigned to you. This will come into use the year after your bankruptcy. Should your record become corrupted or duplicated, HMRC will write to you to confirm which UTR you should use.
To register for a new UTR, follow the link to HMRC's website for Register for and file your Self Assessment tax return