If you are curious, Clifford Adams and I originated the double brackets for UseModWiki to support Wikipedia’s initial roll out (on UseModWiki) and their need for “free links” instead of WikiWords. Prior to that I don’t know of any use of that syntax.
Systems following were cloning Wikipedia I assume.
I am glad I am still causing endless chittering about syntax choices twenty years on. I am sure my gravestone will be “[[RIP]]”.
Single brackets were used for numbered or named “footnotes” by the way.
Here is the design discussion.