Prevent User ReEntering Same Data In Fields

i have a table which has many field, they go across and are named as a1a2a3a4a5a6-a16 the next row will be b1b2b3b4b5b6-b16 the last row is g1g2g3g4g5g6-g16 and so on.

now in the first column i.e which would be a1 b1 c1 d1 e1 f1 g1, the user enters a part number what i want to do is prevent him or her entering the same part no.

twice if this the case the two filds with same part no should be cleared.i.e if user enter p45 in a1 then p45 in d1 then both a1 and d1 should be cleared would be better if the later was only cleared but any method would do. Code: