I have extracted some data, which is text information with a unique key.  In some (or most) circumstances the data is formatted on different lines with the same unique key. 

To keep fidelity of the format it was extracted, I want to add a sequential number, restarting when the unique key changes.  Any ideas?

A12  SWITCH: TEMPERATURE
A12  Model 123
A12  Supplier: Acme
B21  Gasket
C54  SWITCH: TEMPERATURE
C54  Model 124
C54  Supplier: Acme
D66  Blah

A12  1 SWITCH: TEMPERATURE
A12  2 Model 123
A12  3 Supplier: Acme
B21  1 Gasket
C54  1 SWITCH: TEMPERATURE
C54  2 Model 124
C54  3 Supplier: Acme
D66  1 Blah

Any ideas?