Can someone sign someone else's signature without their permission?

You can sign FOR someone else.

But then you need to have a document signed by THAT person stating that YOU are allowed to sign for that person. And you sign in YOUR name, also explaining that you are allowed to sign for the other person.

You're NOT allowed to sign AS another person.

You're not that person.

That's fraudulent, or counterfeiting. Even if it's done with their permission.

We asked this question as well and were told that specific language had to be used to prevent future issues with the signing party. A disclaimer similar to the one below should be used. Please make sure you always consult an attorney before implementing this language to verify its validity. I [name] authorize [your name / company name] to use my signature stamp exclusively for [intended use] on behalf of [client company / individual name]. No other use is permitted.