Looking for some Sequence numbering help

Hi
I need help with creating a formula that will number items sequential and also repetitively

Col A: this is the # of cheques
Col B: This is the Invoices in each Cheques ( So basically 1 cheque can have multiple invoices)
Col C & D: these are the columns i need formulas for, I want Col C to automatically number the cheques and i want D to number the invoices in each cheque and the start again from 1 when it is counting a new invoice.
So for example in the table below Cheque "test 130" is 1 cheque with 2 invoices so Col C2 and C3 would be "1", while col D2 =1 and D3=2, ( which represents the 2 invoices in that one cheque
Any help with the formula would be greatly appreciated

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Try this in C2:

=IF(A2=A1,C1,N(C1)+1)

in D2:

=COUNTIFS($A$2:A2,A2,$C$2:C2,C2)

@ FlameRetired,

Yes! It is simpler than I thought .

Thank you both so much it worked