Duplicate withholding tax certificate number

Hi, when the maximun number of the withholding tax certificate numbering group (interrval) was reached, the counter was reset to the initial number of the interval (I supposed because there was no other interval  to use), so I have record with withholding tax certificate number duplicated in the WITH_ITEM-CTNUMBER field .

I created another interval, but I want to update CTNUMBER field of last records with the counter of the new interval, how can I do this?

Another question: how can I avoid it in the future (This is counter reset to interval's low value, when a new interval is missing)?

I hope you can help me

Thanks

Marco