If you start your plants from seed usually the first year you will have a very high germination rate ~100%, however as the seeds age the germination rates decrease due to rot, false starts, cooking, etc. This means that you have to plant more than a single seed in each spot or risk watering a pot that will never grow.
So as a gardener we plant quite a few seeds in a single spot and with our luck more than we expected sprout and begin to grow. A problem that most first time gardeners make is trying to save all the plants, usually with the idea that since they put the effort in they should keep all those plants. The problem with that is unless you planted a single seed in each pod, or only one seed actually germinated then you are going to be disturbing the soil and roots of both plants to separate them. This will cause shock to BOTH seedlings resulting in either early death or stunted growth. So our position as master gardener makes us the judge of what is the strongest of the plants and executioner of the weaker ones.
First step is to determine which plant is the one you want to keep. Check the seed leaves for any discoloration. See which one is standing straight and maybe they have already started with their true leaves (the leaves that come after the first two). Once you determined which one (or two if you want to thin after more true leaves appear) take a pair of scissors and snip the ones that lost out off at he base. If no true leaves have appeared then this will kill the plant before the roots are established enough to send forth new growth. If you do leave a second plant in the pot as in the picture, do not wait longer than the appearance of the second set of true leaves to thin the remaining two plants, otherwise the lost plant will have used more nutrients from the soil that will limit your first plants growth.
One thing to note is that if the plant’s foliage is eatable then you can eat your thinned plants, so not all is wasted. NOTE: In these pictures I am growing tomato plants. Tomato plant foliage contains atropine and tomatine which is poisonous and should NOT be eaten. These plants went to the compost heap.