Multiple plants in one spot

While working with Pips, I found they would not plant in a spot they normally would.  After a while, I realized that one of the spots had two Mealwood plants.  Earlier, I had many more Pips in this area, so they may have been able to add the two Mealwoods simultaneously.  Removing the duplicate plant allowed them to plant on the empty slot to the left, where the Pip is currently.