Dear Benedict

Thining is recommended however if the growths are significant the thinning procedure is not required, generally thinning is done to rectify the seedling/seeds which doesnot show significant growth compared to other seeds sown, therefore i would rather say that by morphological parameters if you an judge its fine, either let them grow and during the harvesting process by calculating the morphological charateristic you can opt for best out of three

Depending on your experiment select based on a clear criteria like keep the highest plant in each pot or assign numbers to each sprout and make a true randomized selection like using a dize or random table.

Select an average sized plant and remove others

Dear all, thanks for your contributions. 

Keep the most rigorous plant in each pot e.g. keep the largest plant in each replicate.

Counting the number of nodes of similar seedlings. Stage of development.