Use submission list from base ref for duplicate URL check

Previously, the duplicate URL check was configured to use the submission list at the head ref, where two ocurrences of
the URL indicated a duplicate. A more secure approach is to use the submission list at the base ref, and this means that
a single ocurrence of the URL indicates a duplicate.

Author: per1234

main.go

@@ -246,7 +246,6 @@ func populateSubmission(submissionURL string, listPath *paths.Path) (submissionT
 
 	// Check if the URL is already in the index.
 	listLines, err := listPath.ReadFileAsLines()
-	occurrences := 0
 	for _, listURL := range listLines {
 		listURLObject, err := url.Parse(strings.TrimSpace(listURL))
 		if err != nil {
@@ -255,11 +254,8 @@ func populateSubmission(submissionURL string, listPath *paths.Path) (submissionT
 
 		normalizedListURLObject := normalizeURL(listURLObject)
 		if normalizedListURLObject.String() == normalizedURLObject.String() {
-			occurrences++
-			if occurrences > 1 {
-				submission.Error = "Submission URL is already in the Library Manager index."
-				return submission, ""
-			}
+			submission.Error = "Submission URL is already in the Library Manager index."
+			return submission, ""
 		}
 	}