What does this "double less than or equals to" sign mean? I found this in a Computer Science pseudocode context (see page 4 of this paper).

 A: It is the same old $\le$ symbol.
Mathematical notation is far from set in stone or standardised, as might appear to a beginner. Here's a sample of the variations in the inequality signs (taken from symbols-a4.pdf): 


Of course, most of this is not commonly used; my guess is that they have been included only for historical purposes and for completeness. 
A: I add a comment for the sake of completeness. As said, in general $\geqq$ means $\geq$, but in some old math texts it is possible to find the following distinction for a real vector $x$:


*

*"Positive" denoted as $x > 0$: all the elements of $x$ are strictly positive;

*"Semipositive" denoted as $ x \geq 0$: all the elements of $x$ are nonnegative but at least one of them is strictly positive;

*"Nonnegative" denoted as $ x \geqq 0$: all the elements of $x$ are nonnegative.


In other terms, with this notation, $x \geq 0$ implies $x \neq 0$.
A: It means exactly the same as $\le$.
A: In the given context, it means “less than or equal to,” just as “≤” means. However, it is coded as a separate character (not just a glyph variant), so it could be used for some other meaning. A symbol means whatever people make it mean.
Historically, it is a glyph variant, and it has been encoded as a character primarily for compatibility reasons, see Unicode Standard, pages 491–492.
According to the ISO 80000-2 standard, the character to be used is “≤” (U+2264).
