I am trying to use data validation to check whether when data is entered if it is repeated. As the data is in a different worksheet I have created a dynamic named range using the formula:
=OFFSET(BMs!$A$6,0,0,COUNT(BMs!$A:$A),1)
called BMs and then used a custom function in data validation to check if the input is repeated using formula:
ISNA(VLOOKUP(B12,BMs,1,FALSE))
This gives me the same result whether an input is repeated or not.

any ideas?