If you are on a server, if you spam move an item into your storage system, the item will duplicate. This is harder to do if you have a good connection to the server, but the laggier the connection, the faster you can duplicate.

much changes in new versions, this appears solved. New issues always welcome https://minecraft.curseforge.com/projects/simple-storage-network/files/2659393

When you say spam-move. what does that mena? Is it shift-clicking them in from your inventory into the GUI? ?

Also what storage is attached to the network,chests or something else that can make a big difference

I experienced the issue when simply clicking on the item in my inventory, moving it above the Gui, then spam clicking until it went in.