Diamondize Some Text
Charcoal, 17 bytes

ＧＨ&lt;↗↘&gt;↖↙ＬθθＧＨ&lt;Ｌθθ


Try it online! Link is to verbose version of code. Explanation:

ＧＨ


Draw along a path.

&lt;↗↘&gt;↖↙


Draw in the directions ↘↗↗↘↙↖↖↙ (the &lt; and &gt; are shorthands for those two pairs, but the other pairs don't have shorthands.)

Ｌθ


Each path segment has the same length, including the ends, of the length of the input.

θ


Use the input as the text to be written along the path.

ＧＨ&lt;Ｌθθ


Print the first two parts of the path again so that the middle character is correct.