it may happen that annotations are loaded twice

[r03ert0]

If you move quickly through slices back and forth, it may happen that slice annotations will be loaded twice. There's code to prevent annotations to land in the wrong slice, and code to prevent annotations to be loaded in a busy slice. However, it may happen that while a slice is still empty, by going back and forth the server gets queried twice, and then the annotations are displayed twice.
The solution would be to check the annotation's uid, and check that there's no annotations with the same uid already present.

[r03ert0]

an easy way of producing the bug is to go to a dataset with a single slice, load it, and push the left and right arrows as many times as you can before the first annotation loads. You'll end up with several overlapping annotations.