United My Christmas Puzzle

Seen after it's takeover by United was this Plaxton Derwent bodied AEC Reliance of Wilkinson of Sedgefield seen in Stockton-on-Tee KPT 875C No.4002, it also brought Leylands into this BTC typical ECW bodied Bristol fleet..

Now I have a puzzle for you as in my Ian Allan British Bus Fleets I have found two lots of United and Durham buses with exactly the same  registration within the 990-999 EHN series.