Formula to copy cell from source column and add +1 for each duplicate found.

[jdooley]

Formula to copy cell from source column and add +1 for each duplicate found in the column. When the value is the source column changes the sequential count should restart.
Source New Column
DDD DDD-1
DDD DDD-2
DDD DDD-3
DDD DDD-4
DDD DDD-5
XXX XXX-1
XXX XXX-2
XXX XXX-3
XXX XXX-4
XXX XXX-5
XXX XXX-6
XXX XXX-7
XXX XXX-8
XXX XXX-9
XXX XXX-10
XXX XXX-11
XXX XXX-12
XXX XXX-13
XXX XXX-14
XXX XXX-15
XXX XXX-16
XXX XXX-17
YYY YYY-1
YYY YYY-2
YYY YYY-3
YYY YYY-4
YYY YYY-5
YYY YYY-6
YYY YYY-7
YYY YYY-8

[Fluff13]

How about
=A2&"-"&COUNTIFS(A$2:A2,A2)

[PaulM100]

If your data is in A, then in B: =A2&"-"&COUNTIF(A$1:A1,A1)+1