Problem description & analysis:

In the Excel table below, the 1st column contains sequence numbers and the 2nd column has duplicate values.

Task: Record the value appearing for the first time in the 2nd column and replace its duplicates with the string "cont.".

Solution:

Use SPL XLL to enter the following formula:

=spl("=((d=E(?))\d.group@1(Date)).(Date=$[cont.]),d",A1:B8)

As shown in the picture below:

Explanation:

E() function converts the data range to a two-dimensional table. group@1 groups rows and retrieves the 1st row of each group. The operator \ finds the difference of two sets.