John: Yes, but...

When people say "Come here once" they put the emphasis on "here"; the word "once" is spoken without emphasis, as if it had no important meaning but was only there for grammatical completeness. Suppose a kid had some strange but probably harmless object in his hand and you wanted to see what it was. You might say, without urgency, "Come here once" in the same tone of voice, and with the same pattern of emphasis as "What is it?"