Are all duplicate account numbers in groups as per your example? If so then assuming the account numbers are in B2 down use this formula in A2 copied down
=IF(B2<>B1,COUNTIF(B2:B$55000,B2),"")
If account numbers are in any order try
=IF(COUNTIF(B$2:B2,B2)=1,COUNTIF(B2:B$55001,B2),"")
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