Dragging a repeatable and increasing series

[Sjcorrea]

Does anyone have any suggestions to do the following. I am trying to generate a unique reference number which repeats x number of times (in this example 3 times) before increasing.

BO-20-100-10001
BO-20-100-10001
BO-20-100-10001
BO-20-100-10002
BO-20-100-10002
BO-20-100-10002
BO-20-100-10003
BO-20-100-10003
BO-20-100-10003
and so forth.

I know how to create this list when each unique value is listed once. The trouble I am having is repeating the unique value x number of times and then increasing. Am I overlooking a very simple method for executing this type of series?

Regards
-Sam

[Ken Puls]

Assuming that A2 holds BO-20-100-10001

Put this in A3 and copy down: =IF(COUNTIF(A$2:A2,A2)=3,LEFT(A2,10)&VALUE(RIGHT(A2,5))+1,A2)