I create a webmap and use publish it on ArcGIS Online. Unfortunately the tiling engine of AGOL creates multiple labels for the same item. How can I prevent this? Same label problems occured, when I created the tiles locally.
You'll want to look at annotation featureclasses to avoid this.
If you are using a dynamic labeling engine in your map document to place labels (as opposed to annotation), you might occasionally see duplicate labels in your cache tiles. These should not occur any more frequently than 4,096 pixels in any direction. You can eliminate all duplicate labels by using annotation. When you use annotation, labels are associated with one point on the map and duplicates do not occur.
Much more detail in this help topic.
Don't know about AGOL, but Tilemill, which uses the mapnik renderer, uses metatiles for this purpose. I guess there should be an equivalent in AGOL. See for explanation: https://tilemill-project.github.io/tilemill/docs/guides/metatiles/