When I try to create a new integer type metafield of type “One value”, it shows me an option to tick “Unique values only” checkbox.
But that should be applicable for “List type”.
Looks like a bug. Normally, it will make sense to have “Unique values only” checkbox it it is “List type”
What you’re seeing is expected! According to our documentation, this capability is specifically limited to single_line_text_field, url, and integer types (not list.number_integer).
The reason for this is similar to the custom IDs where you want to ensure no other products can use the same value (think SKU, serial number, etc).