With certain mod launchers (GDLauncher for example), the mod is registered twice. This is a rare issue, but one I've only ever seen once before.

When there is only one instance in the /mods folder. I have a friend who fixed his mod with this same issue, I can ask him how he fixed it.

Thanks for your report, I used Prism Launcher to test this mod, that didn't have this issue.
I searched online and found several pages about it.
https://github.com/13rac1/Minecraft-Water-Erosion/issues/15
https://github.com/SpongePowered/Mixin/issues/253
https://www.minecraftforum.net/forums/support/java-edition-support/1956864-you-have-mod-sources-that-are-duplicate-within
https://forums.minecraftforge.net/topic/9815-double-mod-bug/
Seems that I have to add a line to build.gradle file. I tried it, and GDlauncher can launch with no problem. I think this issue is fixed now, you can download the latest version.