Write test case that demonstrastes duplicates in annotation spaces

[pbouda]

For performance we commented one line in graf.py, so that annotation are added to spaces without check if they are already in there:

https://github.com/cidles/poio-api/blob/master/src/poioapi/io/graf.py#L409

Is there any case where this might cause problems? Write a test case with an example file that demonstrates when duplicates might be added to the same annotation space.