Our plugin has code to detect duplicate versions and it can show this kind of dialog on the right.
Now it seems to happen, that a few customers run into an issue, where this dialog would show when updating a plugin.
FileMaker has an MBS Plugin version loaded. Then the Install Plugin script step writes the new plugin to the same location. FileMaker loads the new plugin, but the OS sees that the plugin with that path is already loaded. FileMaker gets the old plugin again, calls initialize function and this was detects it was called before.
With version 11.5pr5, we put in a check to prevent this. The new plugin ignores if old and new path are the same, but will complain if you have two copies in different Extensions folders. This should fix trouble for the clients mentioned above.