The set macro is linking two objects together permanently

I am using SugarCube 2.7.2 on Twine 2.0.11

I've narrowed down my error to this odd 'feature'/bug, and recreated it in a fresh story.

<<set $First to {A: 0, B: 0}>>

<<set $Second to $First>>

<<set $Second.A to 1>>

$First.A
$First.B

$Second.A
$Second.B

This is the only thing this story contains.

The output from this passage is:

1
0

1
0

$First.A is set to 1, and I cannot see why.

If I move the line "<<set $Second.A to 1>>" to another passage, $First.A remains 0.

Am I going crazy? Or is this a bug/'intended feature' in either Twine or SugarCube?