While a pip is heading towards a planting location, with a seed in their mouth, another pip can pick up a seed and start heading for the same planting location. While the first is planting the seed, or has recently finished, the second pip can then plant its seed (in my case sleet wheat) on that same tile. Even though there is already a seed there. Resulting in two seeds growing in the same location.
The pips seem to count them as seprate seeds, as they then no longer plant new seeds according to "normal" rules (as covered in "Pip planting: everything you need to know" post from a few years back.)
Have a location where a pip can plant.
Have seeds to plant (in my case it was sleet wheat).
Have more than one pip.
Note: I have seen the double planting happen several times, but it only very rarely'seems to result in double plants. Not sure what the specific requirements are for it to happen. Maybe a timing thing, perhaps if the planting animations happen at exactly the same time?