de.slisson.mps.conditionalEditor: "nextEditor" duplicates next available conditionalEditors with lower priority

In case a custom conditional editor is "overriding" an existing one, the `next-editor` cell seems to duplicate already available conditional editors with lower priorities.

In this example someone created a conditional editor to "override" an already existing one and the result looks like this:

The Inner "border" is the one from the original editor and the outer one is the one from the overriding one.