Why would God tell Adam and Eve to “replenish” the earth in Genesis 1:22 if they were the first humans?

And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth (Genesis 1:28; KJV).

According to some, this verse in the King James Version indicates that Adam and Eve were to refill the planet, implying that that they weren’t the first humans God created but were part of a “second creation.” Many who accept the gap theory believe this. However, take a look at the same verse in the New King James Version.

Then God blessed them, and God said to them, “Be fruitful and multiply; fill the earth and subdue it; have dominion over…