English does have double negatives, and they're grammatically correct when used properly. They simply cancel each other out which differs from Spanish where they add emphasis.
It actually depends on your dialect. Many dialects of English use negative concord [1], which is what exists in Spanish and other languages (not emphasis) instead of having them 'cancel each other out'.
For example, in my native dialect, "I ain't never had no need for none of them fancy toys" means "I never had any need for any of those fancy toys". It has nothing to do with canceling out other negatives or emphasis; if one thing is negative, they all need to be.
Spanish double negatives aren't emphatic, they're 'grammatical'. If you need one ('Nadie lo sabe') you cannot add another one for emphasis, and if you need two ('No lo sabe nadie') you cannot remove one for less emphasis.
For example, in my native dialect, "I ain't never had no need for none of them fancy toys" means "I never had any need for any of those fancy toys". It has nothing to do with canceling out other negatives or emphasis; if one thing is negative, they all need to be.
[1] https://en.wikipedia.org/wiki/Double_negative