If element is present in list!

hello everyone.
I have a simple list like the blocks that I attach where I add elements.
can someone help me to perform the check when I add to see if the same element is already present in the list?
if it is already present I would not like to add a new element while if it is not present I would like to add it.
thanks to anyone who helps me.
image

this doesn’t work

btn1

@RockyD
thanks, i will try your block. thanks