Relation suggestions hidden even when stacking is enabled

[reckart]

Describe the bug
When stacking is enabled on span layers, a recommendation is only hidden if it duplicates an already existing annotation in its label. However, for relation suggestions, the code seems to compare only the endpoints and if there is already a relation at the same position, the suggestion is hidden - even if stacking is allowed and the relation does not match the label of the existing relation.

Please complete the following information:

• Version and build ID: 34.2