I've just done my year end procedure. I've opened a new adjustment batch and one of my enteries is give me a "Error : Duplicate
adjustment number." when the listing is printed. Why?

Assuming this is Adagio Receivables, what version are you running? There was an old problem with the adjustment posting journal where this 'duplicate' error message would be printed if any adjustment had been posted against a customer, rather than one with the same adjustment number. This was fixed in Jan 2004.

It might be that the adjustment number in fact is a duplicate - this number cannot exist for that customer in the current, historical or matching transaction file. Preview the customer transactions report for that customer (and click the 'applied details' option) to make sure this adjustment number isn't there.