Auto extend formula down a column

S

Sandy

I have data in columnB which is automatically inserted from an input sheet
by a macro running via a button.. In column A I have the following formula
which inserts a unique number to that column should a unique item be found
in column B

"=IF(COUNTIF(B$1:B70,B70)=1,MAX(A$1:A69)+1,"")".

How can I have the formula automatically inserted into row 71 with the
formula updated for that row, ie

"=IF(COUNTIF(B$1:B71,B71)=1,MAX(A$1:A70)+1,"")".

Sandy
 
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