Running count of items in a column performed with a dynamic array formula

Hi,

I have a table in A1:B20 where I perform a running count of each item in column A with this trusted old formula copied down from cell B2:

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This formula works fine, but I am looking for an alternative which does the exact same thing, but as an array formula which spills the values from B2 and downwards and expands dynamically with the number of values in column A. The values in column A will come from a dynamic array formula itself, spilling the values from cell A2 and downwards. Can this be done?

Noble Gases	Running count
Neon (Ne)	1
Argon (Ar)	1
Krypton (Kr)	1
Krypton (Kr)	2
Argon (Ar)	2
Xenon (Xe)	1
Neon (Ne)	2
Argon (Ar)	3
Argon (Ar)	4
Neon (Ne)	3
Helium (He)	1
Krypton (Kr)	3
Argon (Ar)	5
Argon (Ar)	6
Krypton (Kr)	4
Krypton (Kr)	5
Krypton (Kr)	6
Radon (Rn)	1
Argon (Ar)	7

Best regards,
Marbleking

Make a table of your data.

If this is not the solution your after, post an excel file, so we can test it.

Please try

=MMULT((A2:A20=TRANSPOSE(A2:A20))*(SEQUENCE(ROWS(A2:A20))>=SEQUENCE(,ROWS(A2:A20))),SEQUENCE(ROWS(A2:A20))^0)

or
=MMULT((A2:A20=TRANSPOSE(A2:A20))*(ROW(A2:A20)>=TRANSPOSE(ROW(A2:A20))),ROW(A2:A20)^0)