Try and see the following step its works properly for Enter choices manually
choice in drop down box
Steps
1.) Add a validation rule.
2.) In condition puts following
"Dropdown name" is equal to "preceding::Dropdown name" or
"Dropdown name" is equal to "following::Dropdown name"
3.) Give the screen tips.
Note: please type
preceding::Dropdown name
and following::Dropdown name
in Use a formula
area