|Keys| ID |    
 ___________    
 |10  |  1 |    
 |10  |  2 |    
 |14  |  1 |    
 |12  |  1 |    
 |12  |  2 |    
 |50  |  1 |    
 ___________    

Hello all, I'm looking for a way to number sequentially the ID column for every duplicate entry restarting the count for each unique Number in the Key field. Please note that the key field may have a variable number of duplicates (could be dozens or more).

take this formula:

=ARRAYFORMULA(IF(LEN(A2:A); 
 MMULT((A2:A = TRANSPOSE(A2:A)) * (ROW(A2:A) >= TRANSPOSE(ROW(A2:A))); 
 SIGN(ROW(A2:A))); IFERROR(1/0)))

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