How to prevent a player from being inserted into a table twice?

So, in an earlier post I asked for ideas about a matchmaking system. I now use a table for players looking for a match in the server. However, if the player clicks the button twice, the function inserts the name twice. How can I prevent this?

If your table in an array, then you can use the table.find function. If this returns true, it means there is already a value in the table.