Settle a stupid argument for me: forbidden vs strictly forbidden

[Sigako]

I've been discussing with a friend whether the expression "strictly forbidden" is completely stupid and redundant because it has the same meaning as "forbidden" or not. The person who thinks it's not also says its opinion is supported by the fact that you can find "strictly forbidden" on many signs, but can you really? in your experience how common are "strictly forbidden" sign vs "forbidden" ?

You forgot "absolutely forbidden".
Simply "forbidden" means that it's not a big deal, you can either do it or not if you want.
"Strictly forbidden" means that it's a pretty important thing, so you definitely should do it, if only to see what all the fuss is about.
"Absolutely forbidden" means it's a grave matter, so you MUST do it, or you'll be regretting it forever.
A little bit of Russian "humor".

Jokes aside, "forbidden" is a discrete/absolute attribute without gradation, and, technically speaking, "strictly" is superfluous.
The distinction is informal, and the actual difference is in the level of importance and implied (or even explicit) threat for breaking the prohibition in question, as well as tolerance for loopholes and excuses.
Actual usage of this distinction is often when the speaker lacks the means to enforce the prohibition effectively and tries (usually in vain) to inflate their authority, convey the importance of the ban and properly intimidate the subject.