I bought a tray of twelve lettuce plants early this spring. A couple of weeks after planting them, I noticed or at least noted as a problem, that most of the cells had contained between two and four plants. I dug and ripped the small side plants out as carefully as possible, some just barely having a bit of root still attached. Stripped all leaves except the newest starts at the tip. Now about two or three weeks later, as I harvested today, noted that all of those separated and saved plants are about as large, and are actually growing more vigorously, than the main plants that were barely disturbed during the separation. The larger plants have been giving a steady harvest however, and the salvaged plants just in the past couple of days have caught up with them.
On the other hand when various flowers and veggies have been seeded two or three seeds to the cell, when separated at the first true leaves stage, almost all do equally well, with no apparent side effects from the separation and repotting into individual containers.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.