Item duplication via hopper minecart

I have the automatic pumpkin/melon farm seen in the screenshot. It has a hopper minecart working below the entire 4x4 grid in the following pattern, were "O" represents the position the screenshot was taken from, and two straight rails at the right are powered rails:
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O

I recently acquired melons, so I wanted to replace 5 out of 8 spots with melons, closest one on the left, and all the right row. I've broken the 4 pieces of glass blocks with a silk touch pick axe (also had efficiency 2, but no other enchantments), collected fallen pieces of the glass, one was picked up by the minecart, replaced the pumpkin stalks with melons, went to the chest below to retrieve the glass, went back up and placed the glass back. I ended up with an extra piece of glass, which is when I took the screenshot..

I can only assume that the glass was duplicated by me and the minecart picking it up at the same time.