Fix issues linking a tag that already exists in the tag list (#6395)

* Add stash-id to existing when linking tag
* Validate id list for duplicates in find queries
* Filter out duplicate ids after linking tag

internal/api/resolver_query_find_folder.go

@@ -6,7 +6,6 @@ import (
 	"strconv"
 
 	"github.com/stashapp/stash/pkg/models"
-	"github.com/stashapp/stash/pkg/sliceutil/stringslice"
 )
 
 func (r *queryResolver) FindFolder(ctx context.Context, id *string, path *string) (*models.Folder, error) {
@@ -49,7 +48,7 @@ func (r *queryResolver) FindFolders(
 ) (ret *FindFoldersResultType, err error) {
 	var folderIDs []models.FolderID
 	if len(ids) > 0 {
-		folderIDsInt, err := stringslice.StringSliceToIntSlice(ids)
+		folderIDsInt, err := handleIDList(ids, "ids")
 		if err != nil {
 			return nil, err
 		}