Bug Description

A minecraft:lead can be duplicated very simply by attaching and detaching it to and from a fence.
When the lead is detach it drops 2 pickupable leads and 1 ghost-like unpickupable lead which disappears upon re-joining the singleplayer world/multiplayer server.

Did this behavior use to work in the previous version?

Yes, I have confirmed it exists on Sevtech Ages 3.1.7 Singleplayer, Sevtech Ages 3.2.0-BETA Singleplayer, Sevtech Ages 3.2.0-BETA Server.
(Have not tested it on Sevtech Ages-3.2.1-BETA although changelog doesn't seem to indicate anything that would fix this issue)

Steps to Reproduce (for bugs)

1. Right-click a minecraft:lead onto a fence to attach the lead to the fence.
2. Right-click the fence again to detach the lead, 2 pickupable leads will drop and also 1 ghost-like unpickupable lead will drop that isn't affected by the Realistic Item Drops mod. (Unpickupable lead disappears after re-joining singleplayer world/multiplayer server)
3. This can be repeated to obtain infinite leads.

Logs

N/A

Client Information

• Modpack Version: 3.1.7 (Also tested on 3.2.0-BETA)
• Java Version: Version 8 Update 271 (build 1.8.0_271-b09)
• Launcher Used: CurseForge BETA
• Memory Allocated: 10240MB (10GB)
• Server/LAN/Single Player: Single Player & Server (Both tested)
• Optifine Installed: NO
• Shaders Enabled: NO

World Information

• Modpack Version world created in: 3.1.7 (Also tested on 3.2.0-BETA)
• Additional Content Installed: NONE

I can confirm this. This happens for me as well

There is another way to achieve this. Take a horse, a fence, and a lead. Attach the lead to the fence, then to the horse. Now, the lead will appear to both connect the fence, the horse, and you (if you move, the horse will follow. IMAGE)

After this, click the fence to release the lead. You will still be holding a lead, which you can then attach and detach from the fence, resulting in 1 extra lead per use. As with the other dupe glitch, this will result in a ghost/unpickupable lead as well as the 2 pickupable leads.

I haven't tested this with mobs other than horses.