re: '¡No pare, sigue, sigue!'
I understand all of that, my confusion was as to why one verb was an Ud. command ("No pare") and one was a "tu" command("sigue"). However, that assumes they are both true commands (I figured so because of the context) and not just part of an idiomatic expresion, in which case anything goes! What javero was saying also makes sense, but I guess I wasn't expecting to see it on the shirts since the s-dropping is a spoken nuance, and it's not universal.