Clones also react to broadcasts! So, the following code…

when green flag clicked
set [score v] to [0]
create clone of [myself v]
broadcast [increase score v]

when I receive [increase score v]
change [score v] by (1)

will actually increase the score twice!

To fix this, we can use a variable set to for this sprite only to keep track of what is a clone, and what is the original sprite. In the following code, the is clone? variable is set to for this sprite only.

when green flag clicked
set [score v] to [0]
set [is clone? v] to [1]
create clone of [myself v]
set [is clone? v] to [0]
broadcast [increase score v]

when I receive [increase score v]
if <(is clone?) = [0]> then
change [score v] by (1)
end

I hope this helped!