Have a look at https://www.servicenow.com/community/platform-analytics-articles/get-duplicate-records-from-any-tabl.... This way you can get the existing duplicates out of the system.

If the index is not being created, because it says non-unique, there must be a duplicate somewhere.

As background script, delivering something like :

*** Script: INC0008111,INC0009009

gs.info(getDuplicates('incident','number')); //your table and fieldname

function getDuplicates(table, field) {
	var dupRecords = [];
	var gaDupCheck = new GlideAggregate(table);
	gaDupCheck.addAggregate('COUNT', field);
	gaDupCheck.addNotNullQuery(field);
	gaDupCheck.groupBy(field);
	gaDupCheck.addHaving('COUNT', '>', 1);
	gaDupCheck.query();
	while (gaDupCheck.next()) {
		dupRecords.push(gaDupCheck[field].toString());
	}
	return dupRecords;
}