If your mod chest is self duplicating, try this.

Type in /kill into Minecraft. That will cause you to respawn ( the mod chest should then reappear).

Log out of Minecraft, and remove your Minecraft username and re-add it (remember, capitalization matters! Your Minecraft username must match what you see on your Minecraft launcher exactly). To do this, click the Account gear icon in the middle of the LearnToMod homepage and scroll down until you see your Minecraft Username. Then press change. Then log back into Minecraft.

You may need to force shutdown your server a few times, as well.