Personal chest's BUG. Double the item. like this #5027.
My English is not very good, please forgive me. You can also watch this video, don't know if you can open it, this video is in China.
https://www.bilibili.com/video/av39131440

modpack SkyFactory4 Mekanism-1.12.2-9.7.2.373.jar in a server

1.Get two personal chests in your hotbar.
2.Open box at number 4 and press button number 3 quickly
3.When the speed is fast enough, the item in box No. 4 will be displayed, but the box No. 3 will be opened, and the item in box No. 4 will still be taken out.