Google Spreadsheets: Group rows by equal cell and set background color for these rows

Question

Using Google Spreadsheets: I need to Group rows by equal cell value and set background color for these rows.

I need to alternate two color, one for each match.

Here an editable spreadsheet with the real example. The key value is the cell on column B:title (KEY)

Editable file: https://docs.google.com/spreadsheets/d/1klXxhhdgaazGc-secQPZfwdNijLUK6OVOQUxdU-scQU/edit?usp=sharing

Capture:

Answer 1

Added solution to your sheet here:

Helper Column formula:

=scan(;B2:index(B:B;match(2;1/(A:A<>"")));lambda(a;c;if(c<>offset(c;-1;);a+1;a)))

Conditional formatting custom_formula:

Apply to range: A2:C

=isodd($D2)
=iseven($D2)*len($D2)