He just said "double bow" before, but why did he say "thus a double bow" again? I wonder why "thus" was used here.

Rebecca

He repeats "double bow" here to show that he has completed the "double bow tie". "Thus" is an adverb used here to mean as a result. After he has explained that you must tie a "double bow", he says "and thus, a double bow" to show that he has demonstrated how to tie a "double bow" and as a result, he has a completed "double bow". It is common for words to be repeated in this manner when someone is demonstrating something visually.