As long as the seedlings aren't too close together (as in touching) and their roots aren't tangled with each other it won't harm the other plants to pull them up. But if you are afraid of damaging the other plants, just snip off the extra seedlings at soil level with a pair of scissors. The snipped seedlings won't regrow.

Rodney

Pinching off with fingernails also works.

You don't have to thin all at once to the mature spacing. Baby onion sprouts and spinach are not only edible but a delicacy in salads, so you can thin gradually until your plants become full sized.

I would look at it this way:
What is the diameter of an onion ? 3 inches " then i will grasually thin them so they are about 3" apat.

What is rh e diameter of a carrot? ONE inch : Then thin them one inch apart.

I will let the spinach grow in clumps. Thou you can tin and eat them as thin them. Same goes for things like parsley, coriander, chive. Let them clumpup. That is ok.

SNIPE them with your finger nails. Or just pull after watering.