A small discovery is not small to the person discovering it for the first time.

The size of a discovery is measured from where the person stands, not from the top of what is known. To someone meeting an idea for the first time, the ground has genuinely shifted. Meeting that with a shrug or a note that everyone knows it already does nothing for accuracy and quite a lot to shrink the person.

If somebody tells you something you learned years ago, ask what they noticed rather than confirming that it is common. Let the delight belong to them for a minute. You can add what you know afterward, once it will sound like company rather than correction.