Use Case 1:

Given the user can move a column to the first position in the table with a simple drag/drop
When in the first column consecutive values are identical
Then those consecutive-identical values are vertically merged into one cell

Use Case 2:
Given the user can move the position of a given column in the table with a simple drag/drop
When the values of the previous columns are merged
Then the consecutive-identical values of the next column are vertically merged into one cell