ExampleAdd_duplicate: Fix ineffassign warning
ExampleAdd_duplicate: Fix ineffassign warning

We are not using the result of the second Add call in the duplicate
test, which is expected to panic, so there is no need for re-setting
the variable's value.

Signed-off-by: Stephan Peijnik <6ff875d5fb2cf957858f4aac3650c8acf54e0adf@gmail.com>

speijnik/go-errortree