hello @FrankZiyar and happy new year all… 
I dont know anything about Integromat .
but this would be fairly straight forward. If you use the “Uppercase” example integration tutorial as a base. Note: I didnt test this so its not a working solution. Just the building blocks.
Trigger on your target column. (as stated.)
Use the Monday sdk with graphql. in my example “dept” is the target.
you will have to set this up in your code. below is just from playground.
query {
boards (ids: 8825xxxx6) {items {id name
column_values (ids: “dept”) { value }
} } }
Then in your code
let targetlist = [ ];
data.forEach(item => {
targetlist.push(item.column_values[0])
})
let seen = new Set();
var hasDuplicates = targetlist.some(function(currentObject) {
return seen.size === seen.add(currentObject.value).size;
});
then “hasDuplicates” needs to be false if your target is good. or replace text with your warning text on true …
hope that helps.
cheers.
Eric.