Answer: Seedlings should be thinned as soon as they are large enough to handle, and they will need additional thinning as they grow or are moved to individual cells or pots. The reason for this is they need lots of light and air to grow well and stay healthy and will also compete with each other for water and nutrients if kept too close together.
If your seedlings are still very close together and are that tall, it may be a bit late to thin or transplant them, but you can try. Some gardeners will use a small pair of scissors to snip them off at the soil line -- in my experience this works well because leggy seedlings can be a bit fragile and you don't want to accidently damage the ones you want to keep.
Good luck with your cornflowers!
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