Repeat the Nth elements
Jelly, 3 bytes

Ḣs×


Input is taken as s, n.

Try it online!

How it works

Ḣs×  Main link. Argument: s, n

Ḣ    Head; yield s.
     This pops the list, leaving [n] as the main link's argument.
 s   Split s into chunks of length n.
  ×  Multiply each chunk by [n], repeating its first element n times.