In most cases, the problem occurs when you have the same plug-in installed in multiple locations. If you only have one instance of your plug-in, and it is still reported as being installed twice then it is most likely that 4D and your database structure are placed in the same folder.

4D loads it once because the mac 4dx folder is located next to the 4D.exe file and then it loads it again because it is also next to the structure file.

For example:
C:\All\4D.exe
C:\All\mydb.4db

To resolve this, you will need to create one folder for 4D and a seperate folder for your database.
For example:

C:\MyApp\4D.exe
C:\MyDatabase\mydb.4db