I was playing 1.16.40 when I was practicing ice boating in a ice biome but when a stray got in I went into the water but duplicated then the boat duplicated into another boat.

When you are in a boat, ride the boat until you have a stray in the boat. Then go put the boat in the water and then it duplicates the boat. It duplicated the boat 1 time and then it duplicated the stray itself.